Determining NPSH using System Syzer

OK, There are already lots of articles about NPSH – some of them are pretty darn good including one from Bell & Gossett’s Gil Carlson. This discussion, however, is the practical application of these concepts using Bell & Gossett’s System Syzer tool. Version 4 of System Syzer for Windows® introduced an NPSHa calculator: If you don’t have it yet, you need to get it. It is free and available from www.xyleminc.com/ESP.

NPSH Primer

NPSH stands for Net Positive Suction Head. Gil Carlson joked that it stood for “Not Pumping So Hot”. You can take it two ways: The first being that the pump is not working so well. The other take is you can solve pump trouble by reducing the temperature of the liquid. Either works.

NPSHa > NPSHr

A system designer needs to determine the NPSH available to a pump (NPSHa) and compare that to the NPSH required by the pump (NPSHr). NPSHa must be greater than the NPSHr. If it is lower, the pumped liquid will vaporize or flash within the pump. When these bubbles move through the impeller and reach a higher pressure they will suddenly collapse. This condition is known as “cavitation” and will cause severe pump damage such as impeller destruction, broken shafts or seal failure.

If you have ever stood next to an operating pump and it sounded like it was pumping marbles, you were probably experiencing pump cavitation first hand.

Vapor Pressure

Vapor pressure is a key concept in determining NPSH. Here is how a chemist would define vapor pressure: “All liquids form vapors which exert pressures characteristic of the materials. The pressure exerted by these vapors in the presence of the liquid is called vapor pressure The boiling point of a liquid is the temperature at which its vapor pressure is equal to the external pressure of the surface of the liquid”. God bless chemists.

As an HVAC system designer, this is what we care about: The absolute pressure at which a liquid will boil using the temperate of the liquid at the pump inlet

Let’s look at water. Specifically, let’s look at an uncovered pot of water sitting on a camping stove an ocean beach in Miami. A pressure gauge on that pot would show a reading of zero (0 psig). The absolute pressure on that pot of water would be 14.7 psia – typical sea level air pressure. The water in that pot will boil at 212˚F. We could then say the saturated vapor pressure of 212˚F water is 14.7 psia.

Now, now let’s consider water at 202˚F. We can make that water boil if we subject it to a vacuum. On that same beach, if we put a lid on the pot and attached a vacuum pump, the absolute pressure at which that 202˚F water would boil would be 12.2 psia (5.1” mercury vacuum). So we can say the vapor pressure of 202˚F water is 12.2 psia.

as long as we keep the pressure on the surface of that water above 12.2 psia, it will remain in its liquid state and not flash to steam. You should now see the value of knowing vapor pressure in a pumping system. For 202˚F water, we need to make sure the pressure on that fluid never drops below 12.2 psia as it travels through a pump. If it does, the pump will not feel so well.

NPSHr

How do we know how much the pressure will drop as the liquid travels through the pump? Pump manufacturers tell us. They show it on their published pump curves as NPSHr: The required NPSH to keep the pump from cavitating. NPSHr is shown in feet of head. All our discussions so far have been in units of PSI so we will have to convert them.

NPSHa

As a system designer, we must calculate the “total head” of a fluid at the pump suction minus its vapor pressure. We call this resulting number Net Positive Suction Head available Again, we need to ensure that this NPSH available is greater than the NPSH required by the pump.

The total head is made up of two parts: The pressure head and the velocity head. The pressure head is the potential energy in the fluid system while the velocity head is the kinetic energy. When you are “designing” a pumping system and calculating NPSH, you do NOT include the velocity head. If you are taking a reading of an operating pump, it is appropriate to add in the velocity head for accuracy – although the effect of velocity head is quite small.

Calculating NPSHa is all about reference points. Our goal is to find NPSHa at the pump suction. In order to calculate that value, we usually have to start at a point somewhere else in the system. We can find the NPSHa at the pump suction using the following equation:

NPSHa = ha ± hzhfhvp

ha = Absolute pressure at liquid surface level (open system) or compression tank connection (closed system)

hz = Elevation difference of that location above or below pump suction

hf = Friction losses from that location to the pump suction

hvpa = Absolute vapor pressure of the liquid at its temperature at the pump suction

Again, if we are evaluating an existing pump system using a pressure gauge, we would also add the velocity head.

EXAMPLE #1: EXISTING PUMP INSTALLATION

Let’s consider a pump at a location near sea level where we are pumping 170˚F water. Conveniently, there is a gauge on the suction side of the pump showing a reading of 5 psig. The suction piping is 4 inch steel schedule 40 and the flow rate is 300 GPM. What is the NPSH available to the pump?

Since this is an existing system, we can use the pump suction gauge to determine the pressure head directly: We do not have to start at a reference point somewhere else in the system.

We need to convert the pressure from PSI to feet of head. We do that using the following equation:

Pressure Head (feet) = Pressure (PSI) * 2.31 / Specific Gravity

For our example, the Pressure Head = (5 + 14.7) * 2.31 / 0.98 = 46.6 feet

The vapor pressure of 170˚F WATER = 6.1 psia. Converted to feet of head, it is equal to 14.4 feet.

Velocity Head = V2 / 2g

V= Velocity

g = gravity constant (32.174 ft/sec2)

The velocity of 300 GPM through 4 inch pipe is 7.5 feet / sec. The resulting velocity head is 0.9 feet.

The NPSHa would then be 46.6 + 0.9 – 14.4 = 33.1 Feet. We could also add in the static head due to the height difference of suction eye of the impeller to the pressure gauge port. For sake of simplicity, we will leave this small number out.

Solving Using System Syzer

We can do these same calculations very quickly using System Syzer which has the fluid properties we need built-in.

The first step in System Syzer would be to change the fluid to 170˚F water. System Syzer shows the vapor pressure of 170˚F water as 6.1 psia.

The next step is to find the suction pipe velocity. This is done under the Flow/Pressure Drop scale by picking 4 inch steel pipe and entering 300 GPM.

 

 

 

 

 

 

 

We next go to the NPSHa scale. We need to enter the 5 psig pressure reading so we need to choose a closed system. Since we are taking the reading directly at the pump suction, we enter the static height difference and friction losses as zero.

We check the option to include the Velocity Head in our calculation. System Syzer returns a value of 33.1 NPSHa. We can now compare this value to the published pump curves to ensure proper pump operation.

Denver Omelet

What happens if this pump is located in Denver instead of on a beach? Well, the pot of water talked about above will boil at a lower temperature. Water boils at 202˚F in Denver. So, it takes longer in Denver to hard boil an egg and a pump will cavitate at lower temperatures compared to Miami.

So what happened when we changed locations? Did the properties of water change? No. Water is still water whether you drink it in Denver or in Miami. What changed is the atmospheric pressure.

In Miami, water in an open vessel would have a surface pressure of 14.7 psia and would boil when the water reached 212˚F. We are able to get 202˚F water to boil in Miami by inducing a vacuum of 5.1” Hg (12.2 psia). In Denver, the normal atmospheric pressure is considered to be about 12.2 psia. An open container of water in Denver would then be expected to boil when it reaches 202˚F and it does.

EXAMPLE #2: PUMP INSTALLATION AT 5,000 FEET ALTITUDE

Let’s use the same example above, but this time we will use a location 5,000 feet above sea level.

The gauge pressure doesn’t change. It will still be 5 psig. The absolute pressure will change. At sea level, the absolute pressure at the pump suction is 19.7 psia (5 + 14.7).

At 5,000 feet, the absolute pressure will be 17.2 psia (5 + 12.2).

Our pressure head then becomes 17.2 * 2.31 / 0.98 = 40.4 feet.

Our NPSHa is then 40.4 + .9 – 14.4 = 26.9

As expected, the same configuration in Denver has a lower NPSHa compared to Miami.

 

 

This is first of a three part article. Next month will cover determining NPSH when designing an open system such as a cooling tower application.