NET POSITIVE SUCTION HEAD—NPSH
Mark Brien,
General Manager,
Penguin Pumps
This phenomenon can get complicated if allowed to do so and is a subject about which complete books have been written. So let’s just accept the premise that every impeller requires a minimum amount of pressure in the liquid being supplied in order to perform without the liquid being pumped vaporizing inside the pump, which we may simply define as cavitation.
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Cavitation difficulties can reduce a pump’s performance while causing it serious damage. There will be a marked reduction in head and capacity with the pump often spurting liquid at an erratic flow rate from its discharge pipe, or even a complete failure to operate. Excessive vibration can occur when sections of the impeller are handling vapor and the other sections handling liquid. Probably the most serious problem is pitting and erosion of the pump parts, resulting in reduced life. This is caused by the collapse of vapor bubbles as they pass to the regions of higher pressure. This type of Cavitation is often accompanied by excessive noise and vibration. As the vapor bubbles collapse, the adjacent walls are subjected to a tremendous shock from the inrush of liquid into the cavity left by the bubble. This shock actually flakes off small bits of metal or plastic and the parts take on the appearance of having been eroded. This erosion shows up not at the point of lowest pressure where the bubble is formed, but further downstream where the bubble collapses.
To understand the occurrence of cavitation, it is important to remember that a liquid will vaporize at a comparatively low temperature if its pressure is reduced sufficiently. The pressure at which a liquid will vaporize is called its vapor pressure. From this we can see that a reduction in pressure on the liquid can cause a liquid to vaporize if it is close to its vapor pressure. The relative pressure on the liquid entering into the entrance section of a centrifugal pump is reduced from entrance loss, turbulence, and loss at impeller vane tips as it moves from the suction entrance to the point at which it receives energy from the impeller from its vanes pushing from behind, increasing the pressure until it reaches full discharge head.
We call a proximity of the liquid to its vapor pressure its “available NPSH” (NPSHa) and the pressure reduction inside the pump, the “required NPSH” (NPSHr). When the NPSHa >NPSHr, the pump will not cavitate. The NPSHa is supplied from the system and is solely a function of the system design on the suction side of the pump. Consequently, it is in the control of the system designer. The biggest mistake that can be made by a system designer is to succumb to the temptation to provide only the minimum NPSHr at the rated design point. This leaves no margin for error on the part of the designer, or the pump, or the system. Giving in to this temptation has proved to be a costly mistake on many occasions.
Furthermore, many often believe that the the term NPSHa is equivalent to HS (HS=NPSHa), the vertical distance of the liquid above the center line of the pump impeller. However, in reality, this is far from the truth.
The NPSHa can be calculated by the use of the following formula,
Equation 1: NPSHa = Hpsa + Hss – Hfs – Hvap
Where,
Hpsa = Suction surface absolute pressure (feet of liquid), on the surface of the liquid from which the pump takes its suction. This will be the atmospheric pressure for an open tank or the absolute pressure above the liquid in a closed tank.
Hss = Static suction head (feet of liquid). This is the height in feet of the liquid surface in the suction tank above (+) or below (-) the pump center line.
Hfs = Friction head loss (feet of liquid) between the liquid surface in the suction tank and the suction flange of the pump.
Hvpa = Vapor pressure of the liquid at the pumping temperature (feet of liquid), absolute.
The total suction head Hs, may be defined as:
Equation 2: Hs = Hss + Hpsa – Hfs
Substituting Hs into equation 1, we get
NPSHa = Hs-Hvap,
which is the mathematical definition of NPSHa.
A couple of observations can be made concerning equation 1. First, that when pumping a boiling liquid, the NPSHa equals the static suction head minus the suction friction head ( Hss - Hfs) because the suction surface pressure and the vapor pressure equalize one another. Secondly, a negative NPSH is a physical impossibility because it indicates that the friction losses exceed the head which is available to overcome them.
From these observations we can conclude that when handling a boiling liquid, the static head must exceed the suction friction head by the amount of the NPSHr.
There are six (6) typical pump installations for which the NPSHa should always be calculated. These are:
1. When the pump is installed an appreciable height above the liquid level;
2. When the pump takes suction from a tank under vacuum;
3. When the liquid has a high vapor pressure;
4. When the suction line is unusually long ( > 15 ft );
5. When the pumping system is at an altitude considerably above sea level ( Denver, CO ).
6. Self-priming pump installations
Generally speaking, there are rarely ever any NPSH pump problems when horizontal centrifugal pumps are employed in flooded, open systems at or near sea level.
The net positive suction head varies as the square of the ratio of impeller speeds in the same manner as the head varies:
NPSH(2) = NPSH(1) * (N2/N1)2
The net positive suction head varies as the square of the ratio of the impeller diameter:
NPSH(2) = NPSH(1) * (D2/D1)2
Both of the above based on the pump affinity laws.
When a system offers insufficient NPSHa for an optimum pump selection or for an existing pump in service, there are several ways to deal with the problem. We cam either find a means to increase NPSHa. Or means to regulate NPSHr, or do both.
To increase NPSHa we can:
1. Raise the liquid level—At first glance, this appears to be the simplest solution unless it is impractical because the liquid level is fixed, the amount by which the level must be raised is completely impractical, or the cost of raising a tank or a fractioning tower is excessive. Frequently it will be found that only a few extra feet may permit the selection of a less-expensive or more-efficient pump, and the savings in first cost, energy or maintenance will far outweigh the additional cost incurred.
2. Lower the pump—Just as in the case of raising the liquid level, the cost of lowering the pump may be as prohibitive as one might imagine because it may permit the selection of a higher speed pump less costly and more efficient pump. On the other hand, lowering the pump could subject the motor to get flooded much easier and faster.
3. Reduce piping friction losses—This is recommended under any and all circumstances. Increase the suction line diameter, shorten the suction line length, eliminate 90 degree elbows in the suction line, to name but a few suggestions. The cost of doing so will be easily repaid both by improved suction conditions and by saving in energy. Flow of liquids in pipes—there are formulas which can be used to approximate new flow conditions when system changes are made.
4. Use a booster pump—Very effective for high-pressure services.
5. Sub-cool the liquid — This approach increases NPSHa by reducing the vapor pressure of the liquid being pumped.
6. Increase the impeller diameter
To reduce NPSHr, we can:
1. Use slower speed pumps—The lower the pump speed, the lower the NPSHr. There could be a problem in that a lower speed pump will be more expensive and less efficient than a higher speed pump selected for the same series.
2. Use an over sized pump—because NPSHr by a pump decreases as the capacity is decreased.
3. Use several small pumps in parallel—Smaller capacity pumps require lower NPSHr values. In many cases, three half-capacity pumps of which one is a spare, are no more expensive than one full-capacity pump plus its spare.
The NPSHr curve as shown on a given pump performance curve defines the pressure over and above fluid flash point or vaporization pressure which is needed at the pump impeller eye and takes into account decreased pressures within the pump. It should be noted NPSHr shown on the pump performance curve indicates NPSHr and increases nwith increased flow or water velocity. NPSHr curves are provided by pump manufacturers for specific pumps and are needed because all centrifugal pumps operate at a lower pressure in the impeller eye than the pressure existing at the pump suction flange. The decrease in pressure within the impeller eye is caused by increased water velocity as water enters the working pump parts. NPSH evaluation has extremely limited usage for closed piping circuit and flooded condition ( no suction lift ) applications. It is of great importance on open circuit industrial applications where low suction pressure and/or non-flooded suction lift conditions exist, especially when using volatile liquids because interior pump pressure reduction can cause cavitation and pump damage.
Operating a pump past its duty limit as shown on its performance curve can cause numerous problems that are cavitation related:
1. erratic flow rates
2. pumps tend to spurt out liquid
3. high rate of pump damages—broken shafts, broken impellers
4. turbulence
5. vibrations
6. noisy
7. damage motor bearings
8. reduced flow rates
9. aeration
10. increase impeller tip erosion
11. pump will surge and vibrate
12. broken impellers damage the pump housing
13. eventual complete destruction of the pump
The next subject that should be discussed is “most pump troubles should never happen”, the general rules that will prevent most pump troubles from ever occurring if these common sense rules are understood and followed. These rules are for the novice pump individual and are explained in simple every day terms. Simplicity that works-------at least it did for me.
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