Will Water Go Above Feed Line if Pressurized in a Sealed Environment

Glacial and Periglacial Geomorphology

N.F. Glasser , in Treatise on Geomorphology, 2013

8.6.6 Subglacial Water Pressure

8.6.6.1 Subglacial Water Pressure and Effective Normal Pressure

Subglacial water pressure plays an important role in subglacial processes because it controls the effective normal pressure beneath a glacier. Effective normal pressure is the force per unit area imposed vertically by a glacier on its bed. For a cold-based glacier it is effectively equal to the weight of the overlying ice; thick ice imposes a greater pressure than thin ice.

This is summarized by

$N=pgh$

where N is the normal effective pressure, p the density of ice, g the acceleration due to gravity, and h is the ice thickness.

If water is present at the glacier bed, however, the effective normal pressure is reduced by an amount equal to the subglacial water pressure. The greater the water pressure the more it can support the weight of the glacier and thereby reduce the effective normal pressure.

The equation is modified to

$N=pgh-wp$

where N is the effective normal pressure, p the density of ice, g the acceleration due to gravity, h the ice thickness, and wp is the subglacial pressure.

This holds only where the glacier has a flat bed but in reality effective normal pressure is modified by the flow of ice over obstacles (Figure 7). As ice flows against the up-stream side on an obstacle, the effective normal pressure increases by an amount proportional to the rate of glacier flow against the obstacle. Effective normal pressure is also reduced in the lee or on the down-stream side of the obstacle (Figure 7). The pressure fluctuation caused by the flow of ice against the obstacle is, therefore, positive on the up-stream side and negative on the down-stream side. The negative pressure fluctuation on the down-stream side of an obstacle may cause a cavity to form in the lee of obstacle if it exceeds the effective normal pressure at this point (Figure 8). Cavity formation is favored by high subglacial water pressures, which reduce effective normal pressure, and by high rates of basal sliding, which give large pressure fluctuations over obstacles. Theoretical calculations show that cavities can open at sliding velocities of about 9   m per year beneath a thickness of 100   m of ice, whereas velocities of 35   m per year are required with ice thickness on the order of 400   m.

Figure 7. Schematic diagram of the distribution of effective normal pressure at the glacier bed as it flows over a bedrock obstacle. Modified from Figure 7 in Boulton, G.S., 1974. Processes and patterns of glacial erosion. In: Coates, D.R. (Ed.), Glacial Geomorphology. George Allen and Unwin, London, pp. 41–87, with permission from Allen and Unwin.

Figure 8. Small subglacial cavity in the lee of a rock step beneath the margin of the Tschierva Glacier in the Roseg Valley, Switzerland. Ice flow is from left to right over the rock step. The photograph was taken in late summer and the size of the cavity has probably been increased by ablation during the warm summer months.

Subglacial water pressure is controlled by four variables:

1.

Glacier thickness: the greater the weight of the overlying ice, the greater is the subglacial water pressure.

2.

The rate of water supply: inputs of large amounts of meltwater may increase the subglacial water pressure.

3.

The rate of meltwater discharge: an efficient subglacial drainage system will reduce subglacial water pressure.

4.

The nature of the underlying geology: permeable bedrock will allow water to drain through it and therefore reduce subglacial water pressure.

Variations in the rate of water supply and the rate of meltwater discharge are responsible for much of the seasonal variation in water pressure present at some glaciers. Early in the melt season, water pressure may be very high due to the abundance of meltwater and the relative inefficiency of the channel network (Mair et al., 2003). As the subglacial channel network develops during the ablation season discharge becomes more efficient and the subglacial water pressure generally falls.

Variations in subglacial water pressure and its influence on effective normal pressure and cavity formation are very important for the processes of glacial erosion. Subglacial water pressure is also important in determining the rate of basal sliding. Effective normal pressure determines the friction between a glacier and its bed. If the subglacial water pressure rises, effective normal pressure will fall, reducing basal friction, and consequently increasing the rate of basal sliding. This explains why sliding velocity characteristically increases during the summer melt season or after a large rainfall event. Variations in subglacial water pressure have also been linked to glacier surges. For example, the surge of the Variegated glacier in Alaska, during 1982–83 is believed to have been triggered by a change in the subglacial drainage system (Kamb et al., 1985). Prior to the surge the glacier had a subglacial drainage system dominated by a few large tunnels. This appears to have changed to a system dominated by linked subglacial cavities causing an increase in subglacial water pressure due to the lower rate of discharge possible from such a system. This rise in subglacial water pressure facilitated rapid glacier flow during the surge. At the end of the surge this water was released as a large flood and the subglacial system reverted to a large integrated tunnel system. The cause of this change in drainage system is unclear, but it is believed to be central to the rapid glacier flow of this surge.

8.6.6.2 Water Pressure Gradients

The orientation of this network of conduits and tunnels is controlled by the water pressure gradient within the glacier. Water will flow down the pressure gradient from areas of high to low pressure. It is possible to determine the nature of this pressure gradient within a glacier and therefore the direction of water flow within it. Figure 9 shows a hypothetical water-filled tube beneath a glacier. The weight of ice above point A is equal to the weight of the water column BC which it forces up. A line between A and C defines a surface of equal potential pressure. Along this line the pressure due to the weight of the overlying ice is equal to the water pressure it generates. If we now move the tube toward the right, closer to the ice margin, the weight of the ice above point A will fall and consequently the water column BC will be lower. A new lower equipotential surface is defined. Water will flow at right angles to these equipotential surfaces from a surface of higher potential pressure to one of lower potential pressure. As a consequence englacial conduits and tunnels will be orientated perpendicular to the surfaces of equipotential pressure (Figure 10). The geometry of the equipotential surfaces within a glacier is determined by the variation in ice thickness, which is controlled primarily by the surface slope of the glacier and secondarily by the slope of the underlying topography. The surface of a glacier does not always slope in sympathy with the slope of the glacier bed. As a consequence subglacial meltwater may not always flow directly down the maximum slope beneath the glacier and may in some cases even flow up-hill. Under an ice sheet water flow will be approximately radial in sympathy with the surface slope and the direction of ice flow, but will deviate around hills and bumps and be concentrated in topographic depressions such as valleys.

Figure 9. Diagram to illustrate the hydraulic head which drives water flow within a glacier. The weight of the ice above point A is equal to the elevation of the water column BC. The thinner the ice above point A, the less the hydraulic head. Consequently the hydraulic head or potential will fall toward the ice margin or in the direction of glacier slope. Water flows from areas of high hydraulic potential to areas of low hydraulic potential. Modified with permission from Bennett, M.R., Glasser, N.F., 2010. Glacial Geology: Ice Sheets and Landforms, Second ed. Wiley, Chichester.

Figure 10. The pattern of equipotential surfaces within a glacier (i.e., surfaces of equal hydraulic potential). Water will always flow from areas of high hydraulic potential to areas of low hydraulic potential, and therefore it will flow at right angles to the equipotential surfaces as depicted here. Modified with permission from Bennett, M.R., Glasser, N.F., 2010. Glacial Geology: Ice Sheets and Landforms, Second ed. Wiley, Chichester.

It is possible to calculate the water pressure potential at a series of points at the base of a glacier from knowledge of the variation in ice thickness Shreve (1972, 1985a, b). These points can be contoured to define a surface known as the subglacial hydraulic potential surface (Figure 10). Provided that any subglacial tunnel is completely water filled then the tunnel should be orientated at right angles to this hydraulic surface. The subglacial hydraulic potential surface is a useful tool in the interpretation of the glacial landform record (Sugden et al., 1991; Syverson et al., 1994).

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Manned Submersibles, Deep Water*

H. Hotta , ... S. Takagawa , in Encyclopedia of Ocean Sciences (Second Edition), 2001

Water Pressure

Water pressure increases by 0.1  MPa per 10   m depth. Thus every component sensitive to pressure must be isolated from intense pressure changes. First and foremost are the passengers which are protected against great ambient pressure by a pressure hull or pressure vessel, maintained at surface pressure. The ambient pressure exerts strong compressional force on the pressure hull which is therefore designed to avoid any tensile stress. The strongest geometric shape against outside pressure relative to volume and hence weight is a sphere, followed by a cylinder (capped at both ends). However, it is not easy to arrange instruments inside a sphere effectively.

In order to increase mobility, it is important to make submersibles small and light. The pressure hull is one of the largest and heaviest components of the submersible. The hull must be as small (and light) as possible, while affording appropriate strength against external pressure. Thus for deep-diving submersibles, a spherical pressure hull is employed whereas shallower vehicles can use a cylindrical shape if so desired.

The material used for the pressure hull is critical. In earlier vehicles, steel was used. Later, titanium alloy was the material of choice. Titanium alloy has very high tensile strength, and is resistant to corrosion and relatively light (specific gravity ∼60% that of steel). Recently, the trend in submersible construction is to use nonmetallic materials, such as fiber- or graphite-reinforced plastics (FRP or GRP), or ceramics.

Components not sensitive to pressure or saline conditions need no special consideration. Though those devices which require electrical insulation need to be housed in oil-filled compartments called oil-filled pressure compensation systems (Figure 3). These systems do not require heavy pressure hulls and thus reduce the weight of the submersible overall. Electric motors, hydraulic systems, batteries, wiring, and power transistors are all housed in pressure compensation systems. Technology is being developed to apply ambient pressure to electronic devices such as integrated circuits (ICs) and large scale ICs (LSIs).

Figure 3. Pressure resistant and pressure compensated housings for electric motors.

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Introduction to the Physics of Cohesive Sediment in the Marine Environment

Johan C. Winterwerp , Walther G.M. Van Kesteren , in Developments in Sedimentology, 2004

8.1.2 DRAINED AND UNDRAINED BEHAVIOUR

When pore water pressures are generated, two conditions can be distinguished:

drained:	generated pore water pressures are dissipated by pore water flow.
undrained:	virtually no pore water flow through the pores can occur and generated pore water pressures cannot dissipate.

When we consider loading by an isotropic stress increment Δp on a volume of sediment V, the volume balance in terms of volume strain increment Δε_v reads (note that compression is positive):

(8.9) $\begin{array}{l}\mathrm{skeleton}\mathrm{pore}\mathrm{water}\mathrm{solids}\mathrm{transport}\\ \mathrm{\Delta }{\epsilon }_v=\mathrm{\Delta }{\epsilon }_v^{\mathit{sk}}=\mathit{n}\mathrm{\Delta }{\epsilon }_v^w+\left(1-n\right)\mathrm{\Delta }{\epsilon }_v^s+\Phi /V=\\ =\mathrm{\Delta }{p}^{\mathit{sk}}{C}_{\mathit{sk}}+\mathrm{\Delta }{p}^w{C}_s=\mathit{n}\mathrm{\Delta }{p}^w{C}_w+\left(1-n\right)\mathrm{\Delta }{p}^w{C}_s+\mathit{\alpha }\mathrm{\Delta }{p}^{\mathit{sk}}{C}_s+\left({\displaystyle \underset{A}{\int }\underset{¯}{Q}\cdot \underset{¯}{n}\mathit{dA}}\right)/V\end{array}$

in which ε_v is the total volume strain, C_w , C_s , C_sk is the compressibility of pore water, solids and skeleton, respectively, n is the porosity, α is a coefficient for compression of the solids due to skeleton stresses, and Q is the specific discharge of pore water through the surface of V. Note that under drained conditions Δp^w → 0.

The total volume strain increment Δε_v equals the volume strain increment of the skeleton Δε _v ^sk , which comprises:

1.

An increase in effective isotropic stress p^sk , and

2.

An increase in pore water pressure p^w , as a result of which solid particles may decease in volume. This would yield the same decrease in volume of the skeleton

In the undrained case, by definition, the transport of pore water Φ in equ. (8.9) is zero, and the incremental volume change of pore water and solids must balance the total volume change. Two contributions to the volume strain of the solids may be relevant:

1.

Due to an increase in pore water pressure the solid particles may decease in volume, and

2.

Due to an increase in effective stress the volume of the solid particles may decrease non-isotropically (the strain in vertical direction is generally larger than in horizontal directions, which is characterised by a Poisson ratio V ratio smaller than unity).

The isotropic effective stress increment as function of the pore water pressure is found from the balance equation(8.9). Together with the effective stress concept (8.3) the ratio of the isotropic stress increment and the generated pore water pressure increment is obtained:

(8.10) $\frac{\mathrm{\Delta }\mathit{p}}{\mathrm{\Delta }{p}^w}=1+n\frac{C_w-C_s}{C_{\mathit{sk}}-\alpha C_s}$

The coefficient α reflects possible compression of the solids in the skeleton due to skeleton stresses. When the solids consist of spherical particles α is almost zero, while in the complementary case of spherical voids, α equals unity (Schatz, 1976). The latter case occurs in porous rock, and can become relevant in cemented cohesive sediment. In general, α = 0 for un-cemented cohesive sediments.

Typical values for the compressibility of various sediment components are listed in Table 8.1. In general, the compressibility of the solid phase is so small, that only the compressibility of the pore water and skeleton are relevant. When the pore water does not contain free gas, the compressibility of the pore fluid is also small with respect to the skeleton. In that case, an external isotropic stress increment will result in an identical increment in pore water stress. In the undrained case, an external isotropic stress increment has no effect on the effective (skeleton) stresses.

Table 8.1. Compressibility data (Van Kesteren, 1995).

material	phase	compressibility [10−   9  Pa−   1]
water	pore phase	0.48
water + 5% gas	pore phase	100-500
quartz	solid phase	0.027
organic	solid phase	0.5-100
dense sand	skeleton	20
mud	skeleton	1000
soft clay	skeleton	100
stiff clay	skeleton	10
limestone	skeleton	0.55
sandstone	skeleton	0.093

However when gas is present, the compressibility of pore water increases tremendously and an external stress increment generates both a pore water pressure increment and an effective stress increment. The effect of gas on pore water compressibility is discussed in more detail in the next section. In case that also the solids contain gas or compressible organic matter, the solid's compressibility increases and may even become larger than the compressibility of the pore water. In that case the second term in the right hand side of equ. (8.10) becomes negative and an increase in external isotropic stress results in a decrease of the isotropic effective stress. As a result, the sediment becomes very sensitive to liquefaction.

The increase in pore water pressure Δp^w , divided by the pore water pressure increment in undrained loading as a result of a stationary external loading Δp, is shown in Fig. 8.2 for drained and undrained conditions as a function of the Péclet number for pore water pressure dissipation Pe_w . This number is defined as:

Fig. 8.2. Response of pore water pressure to external loading for drained and undrained conditions.

(8.11) $P{e}_w=\frac{V\ell }{c_i}$

in which V is a velocity scale, ℓ is a length scale and c_i is the isotropic consolidation coefficient (subscript ▪ _i is added to distinguish from the vertical consolidation coefficient c_v ). The consolidation coefficient c_i is a material parameter for the dissipation of pore water pressure, defined as:

(8.12) $c_i=\frac{k_i}{{\rho }_{w}g}\frac{1}{C_{\mathit{sk}}-\alpha C_s+n\left(C_w‐C_s\right)}$

in which k_i is the isotropic permeability [m/s] and ρ_w is the water density.

Because permeability and compressibility depend on the isotropic effective stress and stress history, c_i varies with the stress state. However, this variation is much smaller than variations in permeability and compressibility, as c_i is a function of the ratio of permeability and compressibility (e.g. equ. (8.12)).

Fully drained conditions are found near the origin of Fig. 8.2 at Pe_w < 1, represented by a straight line in the diagram, tangent to the full curve.

At high Pe_w (Pe_w > 10) the curve asymptotically approaches fully undrained conditions. The pore water pressure for fully undrained conditions can be computed with equ. (8.10). It is lower than the applied total isotropic stress increment, depending on the ratios of the compressibilities.

It is important to realize that drained conditions do not imply that no pore water pressures are generated: these are always generated, but their magnitude depends on the velocity and length scales within the soil. In sand, drained conditions occur in general. For instance in densely packed sand, shear failure generates dilation of the skeleton, resulting in negative pore water pressures. An increase in shear failure rate and/or size of the deformation zone will generate larger pore water underpressures. These pressures may become so negative that cavitation of the pore water occurs (e.g. Van Os et al., 1985).

Because of the low permeability and high compressibility of cohesive sediment, loading generally generates an undrained response. Such undrained behaviour is responsible for the behaviour typical of cohesive sediment. When the time of loading is not constrained, the behaviour of cohesive sediment becomes drained. An example is the process of surface erosion, which can be regarded as a drained failure process (see Chapter 9).

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Supercritical Water Oxidation

Violeta Vadillo , ... Enrique J. Martínez de la Ossa , in Advanced Oxidation Processes for Waste Water Treatment, 2018

10.1.6 Thermal Conductivity

At constant pressure, water thermal conductivity increases with temperature up to a maximum value around 200°C. Under supercritical conditions, thermal conductivity decreases due to the rupture of the hydrogen bonds. For this reason, heat transfer coefficients are higher around the critical point and diminish above 400°C (Fig. 10.7). Thus, heat losses are higher around the critical point than in supercritical conditions, where losses decrease as a result of the decrease of thermal conductivity.

Figure 10.7. Thermal conductivity (k) as a function of temperature at 250   bar.

SCWO is an oxidation process that takes place above the critical point of water, that is, 374°C and 22.1   MPa. Water polarity is a function of temperature and pressure. At supercritical conditions, water is a nonpolar solvent and it is completely miscible with organics and gases like oxygen (Tester and Cline, 1999). Under these conditions, there is a homogeneous reaction medium, where there are no mass transfer limitations. Furthermore, as the reaction takes place at high velocity due to high temperature used, the residence time necessary to achieve high destruction levels (>99%) are lower than 1   min (Svanström et al., 2004a). Furthermore, the reaction products are not toxic. Production of NO_x, SO_x, and dioxins are negligible because the temperature is too low for these compounds to be produced (Kritzer and Dinjus, 2001).

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Deformation, Storage, and General Flow Equations

Charles R. Fitts , in Groundwater Science (Second Edition), 2013

6.1 Introduction

Changes in head and pore water pressure cause deformation of the solid matrix that holds the water, deformation that has a range of impacts including subsidence, fissures, liquefaction, slope failure, and faulting. Pore pressure changes and matrix deformation are also key aspects of transient (time-dependent) groundwater flow. The last sections of this chapter introduce the general equations of transient groundwater flow, which follow from Darcy's law, mass balance, and storage concepts. The general equations of flow are the basis for mathematical models of groundwater flow, which are discussed in subsequent chapters.

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Surface Waters

Harold F. Hemond , Elizabeth J. Fechner , in Chemical Fate and Transport in the Environment (Third Edition), 2015

Hydrostatic Pressure

In a surface water body, water pressure at a given depth is closely approximated by

(2.19) $P=\mathit{\rho gz},$

where P is the water pressure [M/LT²], ρ is the density of water [M/L³], g is the acceleration due to gravity [L/T²], and z is the depth below the water surface [L]. The proportionality between pressure and depth is a direct result of the fact that water is nearly incompressible, and hence has a nearly constant density. (By contrast, the compressibility of air results in a nonlinear pressure-height relationship in the atmosphere, as discussed in Section 4.1.1.)

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Glacial and Periglacial Geomorphology

K. Yoshikawa , in Treatise on Geomorphology, 2013

8.18.4.1 Groundwater Pressure

Estimated and direct measurement of subpingo water pressure gives values ranging from 100 to 2200  kPa. Müller's (1959) estimated water pressure measurements were between 180 and 250   kPa, and Holmes et al. (1968) suggested water pressure measurements of between 600 and 2200   kPa. Mackay (1998) measured a closed-system pingo (Pingo 14) using a pressure transducer at 22   m below the ground surface, and reported water pressure measurements (taken since 1977) of between 300 and 350   kPa. An (1980) measured an open-system pingo near Kunlun Pass at the northern edge of the Tibet–Qinghai plateau, China, and found that the hydraulic head at the ground surface was from 220 to 320   kPa. Between 100 and 150   kPa of artesian pressure has been measured at Alpha Pingo near Fairbanks, Alaska. The range of values depends on the method of measurement or calculation; however, actual field measurements suggest a range of between 100 and 320   kPa at ground surface. The artesian pressure, which forces water well above the height of the pingo, is commonly 2 or 3 times the atmospheric pressure. Though both open- and closed-system pingos require similar artesian pressure, their source of water and the origin of the pressure are different. Both pingo types form from subpingo water pressure, and the structure (geometry) of the ice/water bodies is similar. As a result, the shape and collapse pattern of the open-system pingo resembles that of the closed-system pingo.

Two of the tallest (highest) known pingos on Earth are Ibyuk Pingo (49   m high) near Tuktoyaktuk Peninsula, Canada, and Kadleroshilik Pingo (54   m high) near Prudhoe Bay, Alaska. Both pingos are approximately 50   m above the surrounding drained lake flats. Pressure at the water lens under these pingos is unknown, but likely exceeds 500   kPa.

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Integrated Sand Management For Effective Hydrocarbon Flow Assurance

Babs Oyeneyin , in Developments in Petroleum Science, 2015

2.10.2.1 Hydrate Formation

Under critical conditions of temperature and pressure water and low molecular weight hydrocarbons such as methane, ethane, propane, and butane are encapsulated within an aqueous hydrogen-bonded structure. The material grows by encapsulating more and more water. The conditions for hydrate formation for methane and water are given in Figure 2.27. This shows that conditions of high pressure and low temperature favour hydrate formation.

Figure 2.27. Pressure–Temperature Curve for Hydrate Formation in Methane–Water System.

Hydrates can form as a simple consequence of temperature decline, which in turn can be the result of local pressure, drops due to turbulence, or gas expansion. The consequence is that even at normal temperatures, it is possible for solid hydrates to continue to grow so that eventually the deposits completely block lines. Prediction of hydrate formation is possible. Knowledge of the hydrocarbon composition, the brine content and salinity, and of the temperatures and pressure profile of the system enables calculation of the risk of hydrate formation and enabling phase diagrams to be constructed showing the hydrate region. Mathematical models have now been developed (e.g., HYSIM/PROVISION) and may include parameters for the common thermodynamic inhibitors, methanol, ethylene glycol, and di- and tri-ethylene glycols.

Salt has an inhibiting effect on hydrate formation (lower gas solubility) so the risks associated with hydrate formation are reduced in high TDS-produced brines.

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Submarine Landslides

Ye Yincan et al , in Marine Geo-Hazards in China, 2017

4.3.1.1.2 Effective Stress Analysis Method

When there is no residual pore water pressure in slope soil, and there is only transient pore pressure (namely, the soil response under the action of wave load is elastic response):

(6.103) $F=\frac{S}{\tau }=\frac{c^{\prime }+{\rho }^{\prime }gz·{\text{cos}}^2\alpha ·tg{\phi }^{\prime }}{\frac{{\rho }_{w}gH2\text{π}\left(z/L\right)e^{-2\text{π}z/L}}{2\text{cosh}\left(2\text{π}d/L\right)}+\frac{{\rho }^{\prime }gz·\text{sin}2\alpha }{2}}$

Of which, c′, ϕ′ are the effective cohesion and effective internal friction angle. For the normally consolidated clay or sand, c′ is approximately 0.

The above formula contains a key assumption, namely when calculating the shear stress under the action of waves on the failure surface of soil, it is assumed that the seafloor surface is horizontal; when τ _xy reaches the maximum value, the soil stress σ _x , σ _y , ｐ solved by the saturated porous elastic medium is 0; see Eq. (6.65).

(6.104) $\text{sin}2\alpha =2·{\text{cos}}^2\alpha ·tg{\mathit{\varphi }}^{\prime }-\frac{{\rho }_{w}2\text{π}\left(H/L\right)e^{-2\text{π}z/L}}{{\rho }^{\prime }\text{cosh}\left(2\text{π}d/L\right)}$

Typically, in the normally consolidated clay and sand, the effective cohesion c′ is 0, the failure surface depth z is much smaller than the wave length, so z/L ≈   /, and the above formula becomes:

(6.105) $\text{sin}2\alpha -2·{\text{cos}}^2\alpha ·tg{\mathit{\varphi }}^{\prime }+\frac{{\rho }_{w}2\text{π}\left(H/L\right)}{{\rho }^{\prime }\text{cosh}\left(2\text{π}d/L\right)}=0$

When there is cumulative residual pore pressure in the slope soil (Fig. 6.38), the safety coefficient is expressed as:

Figure 6.38. Shear failure induced by waves and gravity on infinite side slope.

(6.106) $F=\frac{S}{\tau }=\frac{c^{\prime }+{\rho }^{\prime }gz·({\text{cos}}^2\alpha -r_p)·tg{\mathit{\varphi }}^{\prime }}{\frac{{\rho }_{w}gH2\text{π}\left(z/L\right)e^{-2\text{π}z/L}}{2\text{cosh}\left(2\text{π}d/L\right))}+\frac{{\rho }^{\prime }gz·\text{sin}2\alpha }{2}}$

Of which, r _p   = p _p /ρ′gz is pore pressure coefficient. The cumulative pore pressure p _r can be obtained by Eq. (6.70), and then pore pressure coefficient r _p can be obtained.

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Hydrodemolition Equipment

Andreas W. Momber , in Hydrodemolition of Concrete Surfaces and Reinforced Concrete, 2005

3.2.1 Structure of high-pressure plunger pumps

High-pressure pumps generate the operating pressure and supply water to the spraying device. Generally, they can be divided into positive displacement pumps and hydraulic intensifiers. Positive displacement pumps are standard for hydrodemolition applications. In Germany, as an example, almost 90% of all on-site devices are driven by positive displacement pumps. The most common form is a triplex (three plunger) pump as shown in Fig. 3.3. Major parts of a positive displacement pump are:

Figure 3.3. Structure of a high-pressure plunger triplex pump

(photograph: WOMA Apparatebau GmbH, Duisburg)

•

crank-shaft;

•

pump head with low-pressure inlet valves and high-pressure outlet valves;

•

high-pressure plunger conversion set;

•

pressure regulator valves;

•

switch valves;

•

safety devices.

Life times of pump components depend on many parameters, namely water quality (see Table 3.1), maintenance regime, and operating pressure (see Table 3.3). Most critical to wear and life time is the solid amount in water; this is illustrated in Fig. 3.4. If solid content increases (e.g. due to an insufficient water filter system) cost for replacement parts (valve seats, seals, plungers) increases. Temperature is another critical parameter for pump operation. An increase in temperature increases the probability of mineral precipitation as well as of cavitation. The first aspect is illustrated in Fig. 3.5; a pump part eroded due to cavitation is shown in Fig. 3.6. Both processes are highly erosive to pump components, and temperature control devices, coupled to shut-off mechanisms, should be part of any pump unit.

Table 3.3. Typical life time values for plunger pump components (Xue et al., 1996)

Pressure in MPa	< 30	20 ∼ 31.5	31.5–50	50–70	70–100
Component	Life time in hours
Plunger	2500	2000	1500	1000	800
Seal	1500	1000	750	600	520
Valve	3000	2500	2000	1500	1000

Figure 3.4. Solid content in water and maintenance cost for plunger pumps

(source: Reliance Hydrotec Ltd., UK)

Figure 3.5. Relationship between water temperature and mineral precipitation

(Zentrale für Unterrichtsmedien, Tübingen)

Figure 3.6. Damage to a bearing due to cavitation erosion

(photograph: Neale Cons. Engr., Ltd., Dogmersfield)

The pump head hosts the water inlet and water outlet valve arrangements. It consists regularly of corrosive-resistant forged steel, partly also of coated spheriodal graphite cast iron. Typical plunger diameters for on-site high-pressure plunger pumps utilised for hydrodemolition applications are between 25 mm and 40 mm. The plungers are made from coated steel alloys, hard metals or ceramics (the latter material is limited to rather low operating pressures).

Safety and control devices include safety devices and pressure-measuring devices. Safety devices prevent the permissible pressure from being exceeded by more than 2.0 MPa, or 15%. These devices include pressure relief valves or burst disks, respectively. Automatic pressure regulating valves limit the pressure at which the pump operates by releasing a present proportion of the generated volumetric flow rate back to the pump suction chamber or to waste. It should be used to regulate the water pressure from the pump and is individually set for each operator. Pressure-measuring devices directly measure and display the actual operating pressure.

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