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.