Symptoms of Inefficient Pumping, Acceleration Head’s Effect on Pump Performance & Inlet Piping Requirements

Q. What are some typical indicators of an inefficient pumping system? A. Many pumping system components are not dynamic. These components allow fluid or heat transfer, but aside from thermal expansion or structural vibration, they do not move and do not have dynamic surfaces that wear out. Leakage, fouling, valve failures and cracks in pipe supports are the most common problems. For many industrial facilities, the energy consumed to move fluids makes up a large part of the facility’s overall electricity usage. Operators are often unaware of how to use the energy required to pump fluids. Ineffective energy monitoring may affect a large part of the plant, even the entire facility. A few symptoms in pumping systems often indicate if improper sizing, selection, operation or other issues affect the system’s performance. These include:

• Valves are throttled to regulate flow rate, level and pressure
• Bypass (recirculation) flow regulation
• Batch-type processes, but one or more pumps operate continuously
• Frequent on/off cycling of a pump in a continuous process
• Cavitation noise either at the pump or elsewhere in the system
• Equipment procurement policies based solely on lowest bid price
• A multiple, parallel pump system in which the same number of pumps are always in operation
• A pumping system that has undergone a change in function with no change in pump equipment or operation
• A pumping system with no flow, pressure or power indication

These symptoms do not always guarantee savings, but the more symptoms that are present, the greater the likelihood of potential energy savings. See the Hydraulic Institute’s guidebook Optimizing Pumping Systems: A Guide for Improved Energy Efficiency, Reliability & Profitability for more information. Q. What is acceleration head, and how can it affect pump performance? A. Acceleration head or pressure is a system occurrence associated with both direct-acting pumps and reciprocating-power pumps. It results from the acceleration and deceleration of liquid in the suction piping. Pump users often treat acceleration head or pressure as a loss when calculating net positive suction head available. However, the pressure drop caused by the acceleration is offset by the increase in pressure when the liquid decelerates. Therefore, the acceleration head has a net effect of zero when calculating the average pressure in the suction line. Total suction lift represents the average pressure without reference to the fluctuations above and below this average that result from the inertia effect of the liquid mass in the suction line. With higher pump speeds or with relatively long suction lines, however, this pressure fluctuation or acceleration head must be considered if the pump is to fill properly without cavitation, pounding or suction line vibration. The low speeds of direct-acting pumps normally keep acceleration head low enough for satisfactory operation, but pump users should perform an acceleration head calculation to ensure proper pump operation. With a direct-acting pump, maximum piston or plunger acceleration occurs at the start or the end of each individual stroke. This is reflected in a similar discontinuity in the cyclical pattern of the combined flow curve corresponding to each piston or plunger. The head required to accelerate the liquid column is a function of the length of the suction line, the average velocity in this line, the pump speed, the pump type and the relative elasticity of the liquid and pipe. A similar pressure fluctuation occurs on the discharge side of every direct-acting pump, but analyzing it is difficult because of the pressure influence on the liquid and piping elasticity as well as the length and small diameter of the discharge line in most applications. If low-frequency pressure fluctuation or piping vibration is a problem, a pulsation dampener can effectively absorb the flow variation of both the discharge and suction sides of the pump. For more information on similar topics, see ANSI/HI 8.1 – 8.5 Direct Acting (Steam) Pumps for Nomenclature, Definitions, Application and Operation. Q. What should generally be considered when evaluating a rotodynamic pump’s inlet piping requirements? A. Inlet flow disturbances—such as swirl, unbalance in the distribution of velocities and pressures, and sudden variations in velocity—can be harmful to a pump’s hydraulic performance, mechanical behavior and reliability. Typically, the higher the energy level and specific speed of a pump and the lower the net positive suction head margin, the more sensitive the pump’s performance is to suction conditions. All inlet or suction fitting joints should be tight—especially when the pressure in the piping is below atmospheric—to prevent air from leaking into the fluid. Any valves in the inlet line should be installed with stems in a horizontal orientation to eliminate the possibility of air accumulation. For pumps operating with a suction lift, the inlet line should slope constantly upward toward the pump, with a minimum slope of 1 percent (see Figure 9.6.6.3). For most pumping systems, an inlet shut-off valve should be installed in the suction piping for system isolation. In general, as liquid travels through a piping network, entrained air tends to rise to the highest point. If the pipeline slopes upward, the velocity of the liquid will move the air bubbles toward this high point. In contrast, if the pipeline is fairly flat and the inside surface of the pipe is very rough or if the pipeline slopes downward, the fluid velocity may not be sufficient to keep the air bubbles moving. As a consequence, a pocket of air could collect at high points and gradually reduce the effective liquid-flow area. This reduction in area can create a throttling effect similar to a partially closed valve. Furthermore, a slug of air may be swept into the pump during restart, causing a partial or complete loss of pump prime, especially if the inlet line is kept full by a foot valve at its intake. Any amount of entrained gas in the fluid may adversely affect pump performance. Check with the pump manufacturer to determine the allowable levels of entrained gas. For information on other piping requirements, see ANSI/HI 9.6.6 Rotodynamic Pumps for Pump Piping.