Valves friction losses

The method relies on characterizing the pipe and the subsonic valve by frictional losses in velocity heads. The valve frictional loss is given from equations (7.25) and (7.36) as... 

Type of Fitting or Valve Frictional Loss, Number of Velocity Heads, Frictional Loss, Equivalent Length of Straight Pipe in Pipei Diameters, LJD... 

For fittings and valves, frictional losses should be determined using Eq. (2.10-17) and values from Table 2.10-1. 

The viscous or frictional loss term in the mechanical energy balance for most cases is obtained experimentally. For many common fittings found in piping systems, such as expansions, contrac tions, elbows and valves, data are available to estimate the losses. Substitution into the energy balance then allows calculation of pressure drop. A common error is to assume that pressure drop and frictional losses are equivalent. Equation (6-16) shows that in addition to fric tional losses, other factors such as shaft work and velocity or elevation change influence pressure drop. 

For laminar flow, data for the frictional loss of valves and fittings are meager. (Beck and Miller,y. Am. Soc. Nav. Eng., 56, 62-83 [194fl Beck, ibid., 56, 235-271, 366-388, 389-395 [1944] De Craene, Heat. Piping Air Cond., 27[10], 90-95 [1955] Karr and Schutz, j. Am. Soc. Nav. Eng., 52, 239-256 [1940] and Kittredge and Rowley, Trans. ASME, 79, 1759-1766 [1957]). The data of Kittredge and Rowley indicate that K is constant for Reynolds numbers above 500 to 2,000, but increases rapidly as Re decreases below 500. Typical values for K for laminar flow Reynolds numbers are shown in Table 6-5. 

TABLE 6-4 Additional Frictional Loss for Turbulent Flow through Fittings and Valves ... 

Type of fitting or valve Additional friction loss, equivalent no. of velocity heads, K... 

For determining the frictional head, refer to friction loss in pipes, bends, elbows and reducers and valves as provided in Tables A.I and A.2 ... 

The economics would depend upon the smoother flow of fluid without exce.ssive friction loss. A smaller section of pipe may not only require a higher h.p. for the same suction and lifting head due to greater frictional losses, but may also cause the pipe to deteriorate quickly as a result of the additional load on its surface. Losses due to bends ami valves should also be added in the total friction loss. 

A pipe size incrcascr can be used in the discharge piping. This will reduce the fluid velocity and friction losses. An isolation valve with a low loss characteristic such as a gate valve should he placed after the increaser and check valve. 

Friction losses in straight pipe, valves, and fittings... 

When a PR valve is relieving at rated capacity, the total frictional pressure drop between a vessel and the inlet of the valve should be less than 3% of the set pressure (kPa). In this calculation, the effect on static pressure of fluid acceleration is ignored rather, only friction loss is considered. 

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]). 

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]). 

Control valve loss will be by difference, trying to maintain minimum 60% of pipe friction loss as minimum drop through valve, but usually not less than 10 psi. 

Note that this control valve loss exceeds 60 percent of this system loss, since the valve must take the difference. For other systems where this is not the situation, the system loss must be so adjusted as to assign a value (see earlier section on control valves) of approximately 10 to 20 psi or 25 to 60 percent of the system other than friction losses through the valve. For very low pressure systems, this minimum value of control valve drop may be lowered at the sacrifice of sensitive control. 

The friction losses for fluid flow in pipe valves and fittings are determined as presented in Chapter 2. Entrance and exit losses must be considered in these determinations, but are not to be determined for the pump entrance or discharge connections into the casing. 

The values of friction loss (including entrance, exit losses, pressure drop through heat exchangers, control valves and the like) are hjL and h L. The total static head is D — Sl, or [(D -f D ) — (—Sl)] if siphon action is ignored, and [(D + D ) — (S l)] for worst case, good design practice. 

S] = Friction losses for pipe valves and other system losses, suction side of pump R = Required by pump (NPSH) s = Suction side of pump... 

The absolute pressure at the inlet to the pump is usually the atmospheric pressure in the receiver, plus the static head from the water surface to the pump inlet and minus the friction loss through the pipes, valves and fittings joining the pump to the receiver. If his absolute pressure exceeds the vapor pressure of water at the temperature at which it enters the pump, then a net positive suction hand (NPSH) exists. If this NPSH is above the value specified by the pump manufacturer, the water does not begin to boil as it enters the pump suction and cavitation is avoided. If the water entering the pump is at a higher temperature, its vapor pressure is increased and a greater hydrostatic head over the pump suction is needed to ensure that the necessary NPSH is obtained. 

Determining the total friction loss, however, is not as simple. Friction loss is caused by a number of factors and all depend on the flow velocity generated by the pump. The major sources of friction loss include friction between the pumped liquid and the sidewalls of the pipe valves, elbows, and other mechanical flow restrictions or other flow restrictions, such as back-pressure created by the weight of liquid in the delivery storage tank or resistance within the system component that uses the pumped liquid. 

The number of turbulence or pressure drop producing fittings in the pump suction line should be kept to a minimum. Because of the excessive turbulent and friction loss that they produce, globe valves should not be used in the pump suction line. When NPSH or turbulence is a problem, a turbulence-reducing device should be used. This device should be located as near the pumps suction flange as possible. 

Inlet = 350 psig Outlet = 0 psig Friction Loss = I0 (includes orifice loss) Balance for Valve = 3 0 psi... 

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