Pumping system control valve inefﬁciencies in plant processes offer opportunities for energy savings and reduced maintenance costs. Valves that consume a large fraction of the total pressure drop for the system or are excessively throttled can be opportunities for energy savings. Pressure drops or head losses in liquid pumping systems increase the energy requirements of these systems. Pressure drops are caused by resistance or friction in piping and in bends, elbows, or joints, as well as by throttling across the control valves. The power required to overcome a pressure drop is proportional to both the ﬂuid ﬂow rate (given in gallons per minute [gpm]) and the magnitude of the pressure drop (expressed in feet of head).
For example, for ﬂuid with a speciﬁc gravity of 1, a pressure drop of one pound per square inch (psi) is equal to a head loss of 2.308 feet.
Fluid horsepower = ﬂow rate (gpm) x head loss (ft) x ﬂuid speciﬁc gravity / 3,960 (where 3,960 is a conversion factor)
The friction loss and pressure drop caused by ﬂuids ﬂowing through valves and ﬁttings depend on the size and type of pipe and ﬁttings used, the roughness of interior surfaces, and the ﬂuid ﬂow rate and viscosity. Typical ranges of head loss coefﬁcients (K values) for various ﬁttings are given in the table. Values can vary by 30% to 50% because of variations in pipe size, type of ﬂuid, and other factors. Fitting head losses vary with the square of the ﬂuid ﬂow rate or ﬂow velocity:
HL = K x (v2/2xg)
HL = the ﬁtting head loss, in feet
v = ﬂuid ﬂow velocity, in feet/second
g = the gravitational constant, 32.174 feet/second
K = the ﬁtting head loss coefﬁcient. For valves, K is a function of valve type, size, and the percentage of time that the valve is open.
Energy Savings Opportunities
Pumping system controls should be evaluated to determine the most economical control method. High-head-loss valves, such as globe valves, are commonly used for control purposes. Signiﬁcant losses occur with these types of valves, however, even when they are fully open. If the evaluation shows that a control valve is needed, choose the type that minimizes pressure drop across the valve.
Adjustable speed drives (ASDs) are often recommended for pumping systems that have variable ﬂow rate requirements. When systems are being retroﬁtted with ASDs, the control valve can be removed from the system to eliminate unnecessary pressure drops. The control valve can be replaced with a spool piece or, when isolation capability is desired, a carefully selected low-loss replacement valve.
Figure 1 illustrates the wide variability in frictional head loss as a function of ﬂow rate across three types of fully open, 12-inch valves. Substantial energy and cost savings can be achieved by installing a low-loss valve, such as a butterﬂy valve. When installing a smaller pump impeller, trimming an existing impeller, or making other pumping system modiﬁcations, consider replacing current valves with more efﬁcient ones.
- Check the operating conditions for all control valves in your plant processes.
- Consult vendor catalogs, equipment manufacturers, and DOE’s Pumping System Assessment Tool (PSAT) for valve pressure drop data.
- Use PSAT to estimate the energy losses and costs of throttled valves.
About Jonathon Bell
Jonathon Bell is an entrepreneur, focused on building his family's legacy in the industrial pump market. Currently, he is focused in Latin America, building Dynapro Pumps Mexico from the ground up while contributing in Canada & the United States with Sales & Marketing efforts.
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References: DOE/GO-102007-2228 March 2007 Pumping Systems Tip Sheet #10
Industrial Technologies Program Energy Efficiency and Renewable Energy
U.S. Department of Energy Washington, DC 20585-0121 www.eere.energy.gov/industry
“Control Valve Replacement Savings,” U.S. Department of Energy Performance Optimization Tip, Energy Matters, July 1998; available online at: http://www.nrel.gov/docs/legosti/fy98/23382.pdf