by Terry Henshaw
11/23/2009

The Eleventh in a Series

The Injection of Air to Reduce Noise and Damage

As stated in an earlier article, most pumps are not suitable for handling any significant quantity of free gas. (They are just not good compressors.) The typical centrifugal pump will begin to exhibit a drop in the head‑capacity curve with as little as 1 percent (by volume) of free air in the pumpage, and will be limited in its minimum capacity to prevent the pump from becoming "air‑bound" (2). (The capacity must be kept high to sweep the air through the pump.)
 
In contrast to the above, we have learned that a small quantity of gas, estimated by Karassik (2) at 0.25 to 0.5 percent by volume, injected into the inlet of a centrifugal pump will reduce both noise and damage from cavitation (1). Budris and Mayleben (3) reported an 82 percent reduction of suction pulsations (used as an indicator of cavitation) with the injection of 0.89 percent air into the pump inlet.

The gas may form a cushion to reduce the impact of collapsing vapor bubbles and/or may expand to act against the drop in pressure.

How Much Does an Inducer Reduce NPHR?

Some centrifugal pumps are available with a small axial‑flow or mixed‑flow impeller installed just ahead of the main impeller, as shown in Figure 1. Some are actually installed inside the eye of the main impeller.

Figure 1. A Horizontal Centrifugal Pump with an Inducer

This auxiliary impeller is called an inducer. It requires less NPSH than the main impeller, and increases the pressure and thereby the NPSHA at the eye of the main impeller.

An inducer will typically reduce the NPSHR of the pump (based on a 3 percent head drop of the combined impellers) about 40 percent at the BEP, but will require more NPSH than the main impeller at lower and higher capacities. (It reduces the range of operability of the pump.)

Inducers also normally cavitate continuously, so are subject to a short life when pumping cool water.

NPSH Requirements of a Reciprocating Pump

Why Does a Reciprocating Pump Require NPSH? What Units Should We Use?

In a reciprocating pump, NPSH is required to push the suction valve from its seat and overcome friction losses and acceleration head inside the liquid end. Because a significant portion of the NPSHR is required to open the valve (particularly at low pump speeds) and this is a pressure (rather than a head) requirement, NPSHR for a reciprocating pump is normally expressed in pressure units such as PSI. For example, if a power pump requires 2 psi of NPSH when pumping water (4.6 ft), it will require 2 psi of NPSH on propane (9.2 ft).

Because it is a pressure, some reciprocating pump authorities use symbols for the NPSH characteristic such as NPIP (Net Positive Inlet Pressure) and NIP (Net Inlet Pressure). For simplicity, we will stick with NPSH, regardless of the units used for the quantity.

NPSHR Curves of a Triplex Pump

A study of Figure 2 will provide a better understanding not only of reciprocating pump NPSHR, but also of valve action. These curves are for a 3 in stroke, horizontal, triplex plunger power pump with suction valves that operate vertically. The valves are wing‑guided, each having a seat flow area approximately equal to the plunger area.

Figure 2. NPSH Requirements for a Triplex Plunger Pump

Because the axis of the suction valve is vertical, the valve can operate without a spring if the speed is kept low. Curves A and B represent the NPSH requirements with two different plunger diameters (1.875 in and 2.5 in) tested in the same liquid end with no springs on the suction valves. Note that NPSHR with the 1.875 in plunger at 100 rpm is only 0.7 psi (1.6 ft of water), less than most centrifugals. The NPSH required by the 2.5 in plunger at 180 rpm is only 1.2 psi.

The speed of the pump in this configuration is limited by the ability of the suction valve to keep up with the plunger. Without a spring to push it back on its seat, gravity is the only force tending to close the valve against the entering fluid. If the pump is running too fast, the valve will still be off of its seat when the plunger reverses and starts to reenter the pumping chamber, the liquid will momentarily flow backwards past the valve, and the valve will be slammed onto its seat, sending a shock wave into the suction manifold and piping.

At that instant the plunger is now moving at a finite velocity, but the discharge valve is still closed. The pressure in the pumping chamber will quickly exceed discharge pressure, and the discharge valve will be driven from its seat. A shock wave will be transmitted from the pumping chamber, through the discharge manifold and into the discharge line. The inertia of the discharge valve will carry it beyond its neutral point, compressing the spring more than normal.

As the discharge valve stops, the spring attempts to return it to its neutral position, but again inertia carries it beyond, causing the chamber pressure to rise slightly. This discharge valve hunting is quickly dampened to zero. All of this occurs in a fraction of a second, but can be measured with a pressure transducer in the pumping chamber and displayed on an oscilloscope.

The vertical lines at the ends of curves A and B indicate speeds safely within the range of proper suction valve operation.

Curves C and D are for the same two plungers, but with light springs added to the suction valves. Because the spring force, in addition to valve weight, must be overcome to open the suction valve, NPSHR has increased about 100 percent over curves A and B. These springs do get the valves back onto their seats quicker, though, so that operation is smooth at higher speeds.

If speeds beyond the ends of curves C and D are desired, stronger springs are required. These springs are the standard for this pump, and allow operation in the normal speed range of 300 to 400 rpm. NPSH requirements are about three times that of curves A and B, ranging from about 2 to 4.5 psi.

(Curves A through F represent a pump equipped with the same standard discharge valve spring. Only the suction spring has been changed.)

If operation is required at speeds exceeding the limits of curves E and F, extra strong springs are required on both suction and discharge valves. NPSHR is approximately doubled that of the standard springs, ranging from 4.5 to 9 psi.

It should be emphasized that these NPSHR values are not gauge pressures or absolute pressure. They are pressures above vapor pressure. For example, if the pumpage is deaerated water at 70 deg F (0.4 psia vapor pressure), the above pump with 2.5 in diameter plungers, no suction valve springs and operating at 150 rpm would require a suction pressure of only 1.5 psia (1.1 psi of NPSHR + 0.4 psia vapor pressure), or about 27 in of mercury vacuum at sea level.

Deaerated is the key word because water almost always contains dissolved air. The dissolved air actually increases the vapor pressure of the solution, but is often overlooked when NPSH calculations are performed. The Hydraulic Institute recommends an NPSH margin of 3 psi for power pumps in systems where the pumpage has been exposed to a gas (other than its own vapor). A liquid, such as propane, which is at its bubble point in the suction vessel, requires no such margin.

To minimize the problem of dissolved air, the NPSH tests that produced the above curves were performed with water at, or near, its boiling point in the suction vessel.

What if Air Gets "Sucked" through the Packing?

As illustrated above, reciprocating pumps under the correct conditions are capable of operating with a suction pressure below atmospheric pressure. Such a situation, though, can lead to air being "drawn" through the stuffing box packing and into the pumping chamber on the suction stroke. This air will cause as many problems as air entrained in the pumpage. Capacity will drop, the pump may operate noisily, the system may vibrate and damage may occur to pump and system components.

This inward air leakage can be reduced by an external sealing liquid, such as lubrication oil, directed onto the plunger surface or into the packing. One effective method is to install an endless lip seal packing ring on each side of a lantern ring, with both lips pointing toward the lantern ring, and inject oil with a mechanical lubricator into the lantern ring. Exercise care to prevent over-pressuring the lantern ring because excessive pressure can damage the packing, lubricator and/or stuffing box.

References
  1. Stapanoff, A. J., Centrifugal and Axial Flow Pumps, John Wiley & Sons, Inc., 1948.
  2. Karassik, Igor J., Centrifugal Pump Clinic, Marcel Dekker, Inc., New York, NY, 1989.
  3. Budris, Allan R., and Mayleben, Philip A., "Effects of Entrained Air, NPSH Margin, and Suction Piping on Cavitation in Centrifugal Pumps," Proceedings of the 15th International Pump Users Symposium, March 1998.

 

Pumps & Systems, December 2009



Terry Henshaw is a retired consulting engineer who designs pumps and related high pressure equipment and conducts pump seminars. For 30 years, he was employed by Ingersoll Rand and Union Pump. Henshaw served in various positions in the Hydraulic Institute, ANSI Subcommittee B73.2, API 674 manufacturers' subcommittee and ASME Performance Test Code Committee PTC 7.2. He authored a book on reciprocating pumps, several magazine articles and the two pump sections in Marks' Handbook (11th Edition). He has been awarded six patents. Henshaw is a registered professional engineer in Texas and Michigan, is a life fellow of the ASME and holds engineering degrees from Rice University and the University of Houston. He can be reached by e-mail at pumprof@att.net.