Long Name… Nice Concept…

Over the years, mechanical engineers have established a standardized method for predicting the conditions under which circulator cavitation will occur. Being engineers they were unable to communicate their ideas without the assistance of numbers and formulas. And, after they developed their numbers and formulas, they had to come up with a name that sounded sophisticated. Engineers do this for job security reasons. Hence was born the techno-acronym Net Positive Suction Head (NPSH).

There are two parts to the concept of NPSH. One deals with the conditions created by the piping system and the fluid moving through it. The other deals with the characteristics of the circulator proposed for use in that piping system. We will discuss both of them and see how they work together when determining if a circulator will cavitate.

PIPING SYSTEM FIRST

Although it sounds sophisticated, NPSH is really a pretty simple (and even elegant) concept. In essence it is just a way to predict how far removed the fluid entering a circulator is from its boiling point. If you prevent the fluid entering the circulator from boiling, cavitation won’t occur.

NPSH is a concise way of describing the overall condition of a fluid as it flows into a circulator. It combines the effect of temperature, pressure and fluid velocity into a single number.

The piping system that the fluid travels through affects the temperature, pressure and velocity it has entering the circulator. Because of this, we say that the piping system makes a certain value of NPSH “available” to the circulator. And because five letters are even more impressive than four, the term NPSHA was adopted. The A stands for available.

“Although it sounds sophisticated, NPSH is really
a pretty simple (and even elegant) concept.”

In the context of hydronics, the term “head” means mechanical energy. There are several ways that a fluid in a hydronic system can “possess” this mechanical energy.

Part of the head energy a fluid possesses depends on its pressure. If a pressure gauge were mounted near the circulator’s inlet, its (psi) reading could be converted to a head value by multiplying by 144 and then dividing by the fluid’s density (in lb/cubic foot).

The same fluid also possesses head energy because it is moving. The higher the fluid’s velocity the more “velocity head” it has. To get a number for this you need to find the fluid’s velocity (in feet per second), square that number and then divide by 64.4. If you know the flow rate and the inside diameter of the pipe, you can calculate the fluid’s velocity using formula 1.

 Formula 1where:

v = flow velocity in the pipe (ft/sec)

ƒ = flow rate through the pipe (gpm)

d = exact inside diameter of pipe (inches)

The fluid’s vapour pressure (the pressure at which vapour pockets begin to form) must also get factored into NPSHA. To avoid cavitation the fluid must be kept safely above its vapour pressure.

The vapour pressure of a fluid varies with its temperature.  Figure 1 shows the relationship for water.

The NPSHA concept puts together all of the ways that a fluid can possess head energy. NPSHA can be thought of as the total head of the fluid above the head value at which cavitation will occur. Hence the word “net.” Like any other value for head, NPSHA is expressed in feet of head. If you put all this together mathematically, you get the following formula 2:

Formula 2

Where:

NPSHA= net positive suction head available at the circulator inlet (ft of head)

v = velocity of the liquid in the pipe entering the circulator (ft/sec)

p_i=gauge pressure of the moving fluid measured at the circulator inlet (psig)

p_v=vapour pressure of the liquid as it enters the circulator (psi absolute)

Here is an example: Determine the NPSHA for the piping system shown in Figure 2. Assume the water temperature is 140F.

Solution: First, calculate the velocity of water flowing at 12 gpm through the 1” tube. To do this you need to look up the internal diameter of the tube. For 1” type M copper tubing that internal diameter is 1.055 inches.

Now put the numbers into formula 1:

Next, look up the vapour pressure of water at its corresponding temperature from Figure 1. At 140F the vapour pressure of water is 2.9 psia.

Next, look up the density of water at 140F from Figure 3: 

The density of water at 140F is D = 61.3 lb/ft³.

Finally, put these numbers into the formula 2 and run it through your calculator:

So what does this number of 58.6 feet tell us? By itself it does not tell us much. To make it useful we need something to compare it to. That is where the pump manufacturer comes in.

ANOTHER ACRONYM

Manufacturers test their circulators to find where cavitation begins. They mount a given circulator in a test stand and then reduce the NPSH available at the inlet of a circulator until it cavitates. They add a safety factor and call the resulting number the NPSHR. The R following NPSH stands for “required.” NPSHR is the manufacturer’s specified minimum value of NPSH that must be provided by the piping system to prevent the circulator from cavitating.

The test that circulator manufacturers perform finds the circulator’s NPSHR at several different flow rates. As flow through a circulator increases so does its NPSHR. That is because the faster the fluid goes into the circulator the greater the head loss due to fluid friction inside the circulator’s volute. This head loss pulls the fluid closer to cavitation upstream of the impeller and must be accounted for.

Many manufacturers show the NPSHR values for a given circulator on the same graph as its pump curve. The brown curve near the bottom of Figure 4 is an example.

Figure 4 also shows how you would find the NPSHR of a circulator in a given piping system. First you need to know the flow rate at which the circulator is operating. You do this by plotting the head loss curve of the piping system on the same graph as the pump curve for the circulator. The point where these curves cross is called the operating point. Next, draw a line from the operating point down to the NPSHR curve for the circulator. Draw a horizontal line from this intersection to the vertical axis of the graph and read the required NPSHR value. For the situation shown in Figure 4 the NPSHR is about 2.7 feet.

NPSHR values may not be published for smaller wet rotor circulators. In such cases, use a (conservative) value of five feet of head. Assume this value applies over the entire flow range of the small circulator.

MAKING THE COMPARISON

Now that both NPSHA and NPSHR have been described it is time to use them as a team. This part is really simple: to avoid cavitation make sure the NPSHA provided by the piping system is equal to, or (preferably) greater than, the NPSHR of the circulator.  The greater the NPSHA value is compared to the NPSHR, the greater the safety margin against cavitation. When making the comparison, remember to use the NPSHR value at the flow rate the system is assumed to be operating at.

For the situation shown in Figure 2, the NPSHA was determined to be 58.6 feet. For the circulator shown in Figure 4, the NPSHR at a flow rate of 12 gpm was about 2.7 feet. Since the NPSHA provided by the piping system and fluid is much greater than the NPSHR needed by the circulator, cavitation will NOT occur. In this situation there is a very large safety factor between 58.6 and 2.7 feet. That is good. It helps ensure the circulator operates quietly and efficiently.

My advice is to design all of your hydronic systems so that the NPSHA is always higher than the NPSHR. <>