HI Pump FAQs is produced by the Hydraulic Institute as a service to pump users, contractors, distributors, reps and OEMs.
05/20/2015
Q. What principles drive the pumping action in a controlled-volume metering pump? A. The pumping action is developed by a reciprocating piston. This reciprocating motion develops a flow profile represented by a sine wave. Actual rate of flow is determined by the following formula: Rate of flow = Displacement x Cycles per unit of time x Volumetric efficiency Figure 7.3.1a illustrates how changes in discharge pressure minimally affect the rate of flow from a reciprocating, controlled-volume metering pump.
Figure 7.3.1a. Rate of low versus pressure (Graphics courtesy of Hydraulic Institute)
Figure 7.3.1c shows the rate of flow versus stroke-length setting for a controlled-volume metering pump at a given pressure and stroking speed. The curve is linear, and it is not necessarily proportional. The 50 percent stroke setting may not equal 50 percent flow, because the curve may not pass through zero on both axes simultaneously. Other rates of flow versus stroke length settings can be accurately predicted by measuring flow at two rate-of-flow settings, plotting both points and drawing a straight line through them.
Figure 7.3.1c. Actual flow curve, rate of flow versus stroke length
The relationship between the theoretical and actual rate of flow of a controlled-volume metering pump is illustrated in Figure 7.3.1d. The difference between the theoretical and actual curves represents the volumetric efficiency (VE), the ratio of the actual pump delivery per stroke to theoretical displacement, expressed as a percent.
For more information on controlled-volume metering pumps, see ANSI/HI 7.1-7.5
Controlled-Volume Metering Pumps for Nomenclature, Definitions, Application and Operation.
Figure 7.3.1d. Theoretical and actual flow curve,rate of flow versus stroke length
Q. What are the advantages of dual seals, and how are they subcategorized?
A. Dual seals include more than one set of sealing surfaces. Dual seals typically require the use of an external liquid or gas and are operated with an auxiliary system. They require more auxiliary systems and monitoring than a single seal. The total initial cost is usually higher and requires the installer to have additional training. The benefits are increased reliability and safety.
Dual seals are used when one or more of the following criteria cannot be achieved by using a single seal design:
- Emission control of process fluid to the environment under normal operating conditions
- Emission of process fluid under upset conditions or failure of the primary seal
- Acceptable life (time between repairs) as a result of inadequate lubricating properties of the pumped medium, which may be a result of too many solids too close to vapor pressure, too low viscosity in combination with pressure and speed, or too high pressure differential for one set of faces
- Dual pressurized seals can seal products that crystallize, burn or are otherwise unsuitable for single seals.
- Dual seals can limit loss to the atmosphere. This feature may be a factor in seal selection for equipment handling hazardous materials.
- Seal life may be improved as the seal faces operate in a clean, cool environment.
- Seal performance can be confirmed by monitoring the barrier fluid.
- The faces are lubricated using a clean, cool liquid—usually oil, water or glycol—which provides low wear rates.
- Dual gas seal faces are separated with a clear inert gas such as nitrogen.
- The seal operates independently from the pump. Consequently, loss of product, cavitations or other disturbances can be better tolerated.
- Zero emissions or product leakages are released into the atmosphere.