Startup instructions in many pump instruction manuals indicate the need for each pump to have a union, gate valve, check valve and plug cock. They are recommended for specific reasons.
- The union is the point where the condensate equipment and the rest of the system meet, so the need for a union should be obvious.
- The check valve prevents system problems such as condensate flowing backwards from the boiler or vertical piping when the pump is not operating. The check valve also prevents system problems when one duplex-unit pump is pumping into the discharge of another pump, causing recirculation problems.
- The gate valve, or isolation valve, allows for servicing and isolating the pump from the rest of the system.
- The plug cock or other suitable balancing valve is used to balance the pump.
The plug cock is nothing more than a balancing valve. Originally, “plug cock” was a reference to a square-headed steam cock or any other suitable balancing valve that was typical of steam systems from the 1940s and ‘50s. In today’s world, any good-quality balancing valve with a low flow coefficient (Cv) is acceptable.
The plug cock pulls in the reins on the pump and keeps the check valve from chattering. You need it because manufacturers size most condensate pumps to move more water at 20 psi. That is because a low-pressure boiler can operate up to 15 psig.
Why 20 psi if the boiler can only operate to 15 psig? If the boiler operates at pressure less than 50 psig, you have to pump into it with a pressure equal to the operating pressure, plus 5 psig. So, if your boiler is running at 2 psig, your pump should discharge at 7 psig. Likewise, if the boiler operates at its maximum 15 psig pressure, you’ll need a 20 psig pump to get in.
The problem is, if you let the pump do what it is capable of doing, the pumped flow of return condensate will be much too fast. It will have the check valve chattering like a machine gun.
The lubricated plug cock adds resistance to the flow and stops the chattering. If you had a heating boiler operating at 2 psig, you would throttle the plug cock until you induced about 13 psig of pressure drop in the line. (You need 7 psig to enter the boiler at the proper rate. The plug cock eats up the rest.) You do not necessarily need a pressure gauge to do this -- just listen to the check valve. When it stops chattering, you are probably at the right point.
You cannot throttle with a gate valve because when closed part way, the gate hangs perpendicular to the flow of water. It will rattle back and forth and eventually shake itself off its stem. Besides, you should not use your service valve to throttle: someone will invariably close it to service the pump and then reopen it to its full open position. That would set your check valve chattering again.
NPSH Implications for BalancingJust so we’re all on the same page, I’ll start with some foundational information. In a pump, the impeller or wheel typically does not work the way many people think it does. Those inexperienced with pump internal parts look at impeller vanes and mistakenly think that the vanes scoop the water into the pump discharge, and into the system. Actually, impellers, or pump wheels, slap the water, essentially throwing it to the edge of the pump casing and out the discharge. When this occurs, a low-pressure zone is created in the eye, or inlet, of the impeller. Because high pressure always flows to low pressure, more water rushes into the impeller eye to fill the void created by the pump discharge pressure to the system.
However, in steam systems, net positive suction head (NPSH) comes into play. NPSH is defined as the net positive pressure that causes a liquid to flow through the suction piping to a pump and enter the eye of the pump impeller. NPSH is simply the minimum suction conditions required to prevent flashing of the fluid to steam in the pump.
There are two values for NPSH:
NPSHR -- R for required -- is a function of the pump design. Pump manufacturers provide the NPSHR value to tell users how much NPSH is required to keep the fluid from flashing to steam inside the pump.
- NPSHA -- A for available -- is what is available for the pump to use.
- Positive static head, or water column, of condensate above the pump’s impeller eye.
- Vapor pressure of the fluid.
- Piping friction loss of the water column piping into the pump.
Once the positives and negatives are totaled, the resulting value is what is available. Available NPSH must always exceed required NPSH. If it does not, then the water in the eye of the impeller will flash back to steam. This is called cavitation, and it can damage the pump’s internal components.
Many pump brands are conservative in pump sizing, which means the typical pump within a process heating system or other equipment has the potential to pump far greater than the requirements. A look at a typical pump curve will demonstrate this.
Now, if you are trying to get water into a 15 psi boiler and have only 13 psi coming out of the pump, nothing moves. Until the boiler pushes more steam out and the volume in the boiler and its related pressure drops, no water can get to where you want it -- into the boiler. You need to control the pump.
To think of it another way, the plug cock or balancing valve in a system functions like a nozzle on a garden hose. By using it to increase pressure and decrease flow, the pump moves back on its operation curve, preventing the pump from dead heading against the internal pressure of the boiler or the pressure drop in the pipe between the pump and the next piece of equipment. More than that, though -- as the pump moves back on its operation curve, it moves back on its NPSHR curve, eliminating the potential for cavitation.
Reining in the pump with the plug cock or balancing valve increases the pump’s potential discharge pressure and moves it back to a point on the NPSH curve where the available is greater than the required. Now water is able to reach the boiler or the next piece of equipment in the system, and the condensate pump performs as it should.