Valves, pressure drop across

Figure 15.11 Fluidic characterization of the wafer-level fabricated PPy membrane valve. Pressure drop across the valve 150 kPa.

Cavitation. The subject of cavitation in pumps is of great importance. When the Hquid static pressure is reduced below its vapor pressure, vaporization takes place. This may happen because (/) the main stream fluid velocity is too high, so that static pressure becomes lower than vapor pressure (2) localized velocity increases and static pressure drops on account of vane curvature effect, especially near the inlets (J) pressure drops across the valve or is reduced by friction in front of the pump or (4) temperature increases, giving a corresponding vapor pressure increase. 

Proportional Element First, consider the outflow through the exit valve on the tank. If the flow through the line is turbulent, then Bernoulh s equation can be used to relate the flow rate through the valve to the pressure drop across the valve as ... 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

The simplest continuous-distillation process is the adiabatic single-stage equihbrium-flash process pictured in Fig. 13-25. Feed temperature and the pressure drop across the valve are adjusted to vaporize the feed to the desired extent, while the drum provides disengaging space to allow the vapor to separate from the liquid. The expansion across the valve is at constant enthalpy, and this facd can be used to calculate To (or T to give a desired To). 

Pressure Drop Methods for estimating fluid-dynamic behavior of crossflow plates are analogous, whether the plates be bubble-cap, sieve, or valve. The total pressure drop across a plate is defined by the general equation (see Fig. 14-29)... 

Blockage of liquid Monitor pressure drop across vent system (e.g., effluent line due local indication, alarm or interlock) to closed valves,, Interlock valve in feed line to centrifuge results in flooding of basket and Equipment/line-up checks overflow from Remove unnecessary valves basket to solid col-. 5, lection system in base. Possibility of liquid spill. ... 

When actual mass flow through the expanders is lowered, efficiency varies according to power loss, deflective angle of gas in the leading edge of the rotating blades, and pressure drop across the inlet butterfly valve. The pressure drop increases as the valve closes. 

G = Gas specific gravity = mol. wt./29 Pi = Valve inlet pressure, psia AP = Pressure drop across valve, psi Q = Gas flow rate, SCFH Qs = Steam or vapor flow rate, Ib/hr T = Absolute temperature of gas at inlet, °R T5I1 = Degrees of superheat, °F... 

The soft-seated spring-loaded pilot valve is so constructed as to have a long built-in blowdown. For a flowing type pilot, at the point where the pilot supply line feeds the system pressure to the pilot relief valve, it passes through a variable orifice, which is also the main valve blowdown adjustment. When the pilot opens, the flow through the supply line causes an immediate pressure drop across the orifice. By adjusting the size of the orifice and thus the amount of pressure drop across it, one can obtain any desired system blowdown (5 to 7% is typical). 

Back pressure reduces the pressure drop across the orifice of any type of PR valve. This results in reduced discharge rates in the case of vapors, if the back pressure exceeds the critical flow pressure. For liquids, any back pressure reduces the pressure drop and results in a lower discharge rate. 

We shall first consider the case of non-flashing liquids. In this situation, there is no critical flow pressure limiting the flow of liquid through a PR valve orifice, as opposed to the case of vapor flow. The discharge rate is a function of the pressure drop across the valve and can be estimated by the following expression ... 

Pp = Pressure drop across the valve i.e, the set pressure minus the total back pressure (superimposed plus built-up). It is important to note that although the actual flow is proportional to the square root of pressure drop at relieving... 

In addition to valve velocity, the manufacturer can furnish the effective flow area of the valve. This area is determined by measuring the pressure drop across the valve with a known flow rate and then calculating an equivalent orifice area that provides the same pressure drop. Valves with larger effective flow areas have less pressure drop and better efficiencies. The effects of the seat area, the lift area, and the flow paths are automati-... 

Rather than assuming a pressure drop across the control as 25%, 33%, or 40% of the other friction losses in the system, a logical approach [9] is summarized here. The control valve pressure drop has nothing to do with the valve size, but is determined by the pressure balance (See Equation 2-59 [9]). 

APc = pressure drop across control valve F i = friction pressure drop at maximum flow rate, psi... 

For good control by the valve, the pressure drop across (or through) the valve must always be greater than the friction losses of the system by perhaps 10% to 20% (see [9]). 

Standard flow coefficient for valves flow rate in gpm for 60°F water with 1.0 p.si pressure drop across the valve, = Q (p/62.4) (AP)) ... 

They must be readily accessible for cleaning, which should be carried out as frequently as necessary to ensure that pressure drop across the filter does not affect normal oil flow rates. The filtering medium should be of corrosion-resistant material such as Monel metal, phosphor-bronze or stainless steel. All first-stage filters should be provided with isolating valves. 

If the pressure drop across the valve is to be more than 42 per cent of the inlet absolute pressure the valve selection is the same as if the pressure drop were only 42 per cent. With this pressure ratio the steam flow through the valve reaches a critical limit, with the steam flowing at sonic velocity, and lowering the downstream pressure below 58 per cent of the inlet absolute pressure gives no increase in flow rate. When the heater needs a higher pressure, or when the pressure required in the heater is not known, it is safer to allow a smaller pressure drop across the control valve. If the necessary heater pressure is not known, a pressure drop across the control valve of 10-25 per cent of the absolute inlet pressure usually ensures sufficient pressure within the heater. Of course, in the case of pressure-reducing valves the downstream pressure will be specified. 

When using modulating valves for steam, heat emitter output must be based on the steam pressure downstream of the valve, which often has a high-pressure drop across it, even when fully open. 

The pressure (PSI) for calculating heat generation in a flow control valve, for example, is the inlet minus the outlet pressure, or the pressure drop across the valve. 

The purpose of the regenerated catalyst slide valve is threefold to regulate the flow of regenerated catalyst to the riser, to maintain pressure head in the standpipe, and to protect the regenerator from a flow reversal. Associated with this control and protection is usually a 1 psi to 8 psi (7 Kp to 55 Kp) pressure drop across the valve. 

APC = pressure drop across valve, psi SpGr = specific gravity of fluid, relative to water at same temperature... 

APC = Pressure drop across a coil trol valve, psi... 

Determine the operating point on the pump characteristic curve when the flow is such that the pressure drop across the control valve is 35 kN/m2. 

A microparticulate hplc column is a very efficient filter, and if the mobile phase contains any particulate matter, or acquires it from the pump and/or the injection valve due to wear, it will collect at the top of the column. If this happens, the pressure drop across the column for a given flow will gradually increase, and the column may eventually become completely blocked. To prevent this happening, the mobile phase should always be filtered before use, preferably through a 0.5 p,m porosity filter, and guard and scavenger columns should be used as a matter of routine (see Section 5.3.2). 

A commercial steel (e = 0.0018 in.) pipeline is 1 in. sch 40 diameter, 50 ft long, and includes one globe valve. If the pressure drop across the entire line is 22.1 psi when it is carrying water at a rate of 65 gpm, what is the loss coefficient for the globe valve The friction factor for the pipe is given by the equation... 

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