HEAD, THE REASONS FOR CHANGES IN THE DISCHARGE HEAD OF A PUMP H006
We have to know the discharge head of a centrifugal pump for several reasons:
- To select the correct size pump for a new application.
- To estimate stuffing box pressure in the event the discharge valve is shut with the pump running. This increase in stuffing box pressure can affect the performance of some mechanical seals.
- To determine if an installed pump is running at its best efficiency point (BEP). Any time we go off the best efficiency point (BEP) the shaft is subject to radial side loading that could cause problems with mechanical seals, packing, bearings and couplings.
- To be sure the pump will have enough capacity for the application.
- To prevent cavitation problems if the net positive suction head available (NPSHA) is too low.
There are two methods of determining the head:
- Look at the discharge pressure gauge that has been installed in the system, or a chart recorder that is installed on the discharge side of the pump. Subtract this reading from the reading obtained from a gage installed on the suction side of the pump.
- The chart recorder is the preferred method because it will record the changes in head over a period of time. The gages only reflect the head reading at the time you are looking at the dials.
- Calculate the discharge and suction heads from numbers you calculate after you look at elevation, pressure in suction and discharge tanks, and losses in the piping and associated hardware installed in the lines.
Please keep in mind that these readings will tell you the head being produced by the pump, but that could be very different than the head required by the system
As easy as taking these readings sounds, we can still have many problems determining accurate head information because of variations in the discharge system. In the following paragraphs I will describe how design, operation and maintenance practices can affect this discharge head.
- An automatic or manual control valve is opening and closing to control:
- Level – Boiler feed systems are typical.
- Mixture – Many processes do this routinely.
- Pressure – Common with wash down pumps.
- Temperature – This is very common in heat exchanger applications.
- Flow – Bypassing and flow control are probably the most common applications.
- Two or more pumps are running in parallel:
- A larger pump is closing the discharge check valve of a smaller pump.
- Only one pump is running and it is not creating enough head to stop it from running on either side of its best efficiency point (BEP)
- The wrong size pump was originally installed in the system. This is a common problem in new applications because each individual involved in the selection process has added a safety margin to the original operating specifications. Every one involved in the pump selection has a real fear of buying a pump that is too small for the application. It is also a common practice to purchase a pump that is known to be too large for the application, because of anticipated future needs.
- A storage tank is being filled from the bottom and the discharge head increases as the tank fills.
- The pump is being used as an accumulator because a tower down pipe is keeping a head on the system. The pump comes on when the level in the tank falls to a pre-determined level. The problem is that the tank is being filled from the same down pipe and the head increases as the tank fills. We find this application in municipal water systems.
- The pump motor speed is changing:
- The packing was removed from an induction motor. These motors have a slippage from 2% to 5% depending upon their quality and load. A mechanical seal was installed and the motor has speeded up changing the location of the best efficiency point (BEP) on the pump curve
- A variable speed motor is being used in a system with a high static head. These motors work best where the head is mainly friction or system head caused by the piping, fittings and valves that you would find in a typical hot or chill water circulating system.
- A higher rpm pump has been substituted because of system demands and the piping is offering too much resistance at this increased flow. Keep in mind that the pipe resistance varies by almost the square of the change in pump speed. In other words if you double the speed of the pump you get almost four times the resistance in the piping.
- The system has been altered to accommodate a change in the process. Another storage tank may have been added some distance from the pump, along with more valves, fittings and piping.
- A discharge valve is being closed or throttled to save amperage. This is a common practice when starting centrifugal pumps with lower “specific speed” impellers.
- The product viscosity has changed.
- A more viscous product is now going through the piping. This is a continual problem in pipe-line applications.
- You are pumping a dilatant (Its viscosity increases with agitation). Cream is a dilatant. With agitation it becomes more viscous until it turns into butter.
- If the pump was sized for a higher viscosity product and the temperature has increased, or the product viscosity lowers with agitation (thixotrophic products do this), you can also go off of the best efficiency point (BEP) to the higher capacity side.
- The specific gravity or density of the product has changed causing a change in the rpm of the motor:
- A different product is now in the system.
- A cleaner, steam or solvent is being circulated through the lines to sanitize them.
- The temperature of the product has changed. (specific gravity changes with temperature)
- A discharge valve is being throttled
- A common practice in an attempt to stop cavitation. (The lower the capacity, the less net positive suction head required (NPSHR).
- To lower the liquid velocity in an attempt to compensate for impeller wear or wear on the impeller wear rings.
- To stop water hammer.
- The impeller has been trimmed.
- The discharge of the pump is being restricted.
- A foreign object was left in the pipe the last time the system was opened, or maybe the swing check valve seat has come loose.
- There is a buildup of material on the wall of the piping and fittings. Hard water leaving a layer of calcium on everything is a good example. Many other fluids can coat out on the pipe walls also.
- The pipe has collapsed (often not visible)
- A vehicle ran over it.
- A pipe liner has come loose and is restricting the passage.
- A frozen water jacket has collapsed the pipe.
- A restricting fitting has been substituted or added to the system.
- A reducer was installed.
- A globe valve has been substituted for a gate valve. A globe valve can add another 50 to 100 foot of head depending on the flow and pipe size.
- A gasket is protruding into the piping. The wrong width gasket was installed during a pipe repair or maybe the center of the gasket was not completely removed.
To size a pump properly or to do effective trouble shooting it is important to know where the pump is running on its curve.
There are low cost chart recorders available to help you identify some of these problems and get a clearer picture of what the system is actually doing. I strongly recommend the use of these recorders on both the suction and discharge side of the pump prior to specifying a replacement pump in an existing application.