As we’ve mentioned about 100 times in the past, at Triplex, we specialize in sizing and applying pumps for high purity fluid handling applications. To do this, we need to get all sorts of information about the application and process parameters. One of the most important things we want to know about is the suction conditions of the pump. This is because we need to be able to calculate the Net Positive Suction Head and double check that it meets the Net Positive Suction Head Required (NPSHr) of the pump. So what is suction head? Let’s get go a little deeper!
In addition to the total head of a system, as well as the power and efficiency requirements, the condition at the pump inlet is critical. We can’t pump what isn’t there. We need fluid to enter the pump at a sufficiently high pressure in order to avoid cavitation. This is called the Net Positive Suction Head (NPSH).
Pump manufacturers will supply data about the Net Positive Suction Required (NPSHr) for satisfactory operation of their pump. When sizing a pump or designing a system, we need to make sure that the Net Positive Suction Head Available (NPSHa) is greater than the NPSHr.
The NPSHa is dependent upon the fluid being pumped, the inlet piping, the location of the suction vessel, and the pressure applied to the fluid in the vessel. The sum of these is the actual pressure seen at the pump inlet. Said another way:
NPSHa= Pa + hs – hfs – Pvp
Where:
Pa = Pressure above fluid level (psi or ft.)
hs= Static Suction Head (ft.)
hfs= Pressure drop in suction line (ft.)
Pvp= Vapor Pressure of the fluid (psia)
When feeding the pump from a vented source (open to atmostphere), our pressure above fluid level will typically be 14.7 psi or about 34 ft of head. From there, we need to add (or subtract) any liquid level above the pump. Typically, the pump inlet will be below the fluid level of the tank, so this level helps get product to the pump. But if the pump inlet is above the fluid level being pumped, we will have to subtract this from NPSHa.
From there, we need to calculate the friction loss in the suction line, which is generally pretty low, but should garner careful attention with higher viscosity fluids.
Finally, we’ll need to deduct the vapor pressure of the fluid. We typically find this in a table and it’s a relatively low value, so in many lower temperature applications, can be assumed to be negligible. For higher temp applications, we’ll want to factor it in.
If the NPSHa we calculate is greater than the NPSHr of the pump, all is good and the pump will run without cavitation. If the NPSr is greater than the NPSHa, the pump will cavitate. To avoid this, we’ll have to adjust the system to minimize friction loss through the inlet piping or increase the suction head of the feed fluid.
So when sizing a pump for a new application or troubleshooting an existing installation, start by looking at the Net Positive Suction head of the system and make sure that your NPSHa is greater than your NPSHr. And as always, if you have any questions about your sanitary pump application, contact a Triplex Sales Engineer today.
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