Condensate Pump Start-up and Balancing
Volume 1 / Issue 3 / November 2014
Domestic® Pump often gets requests to clarify piping diagram and start-up instructions, especially the need for each pump to have a union, a gate valve, check valve and plug cock, as shown below. Here’s the reasoning behind our recommendations.
- The union is the point where the condensate equipment and the rest of the system meet.
- The check valve prevents problems like these:
- Condensate flowing backward from the boiler or from vertical piping when the pump is not operating.
- One duplex-unit pump pumping into the discharge of the other pump, causing recirculation problems.
- The gate valve, or isolation valve, lets you service the unit in isolation from the rest of the system.
- The plug cock, or other suitable balancing valve, balances the pump.
Contractors often want to know exactly what a “plug cock” is and why it needs to be installed. We also get questions about seals failing (usually due to cavitation).
The plug cock is simply some sort of balancing valve. The term originally referred to a square-headed steam cock or other balancing valve typical of steam systems from the 1940s and 50s. Today, you can use any good-quality balancing valve with a low Cv, such as the Bell & Gossett Circuit Setter.
In The Lost Art of Steam Heating, here’s what author Dan Holohan says about condensate unit piping and plug cocks –
Put a gate or ball valve after the check valve so you’ll be able to service the pump or the check valve without having to drain the boiler. Then, install a lubricated plug cock or other suitable balancing valve after the service valve. The plug cock pulls in the reins on the pump and keeps the check valve from chattering. You need it because manufactures size most condensate pumps to move more water at 20 psi. That’s because a low pressure boiler can operate up 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. If the boiler operates at its maximum 15 psig pressure, you’ll need a 20 psig pump to get in. That’s why they make them that way.
The problem, however, is if we let the pump do what’s it’s 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 clock adds resistance to the flow and stops the chattering. If you had a heating boiler operating at 2 psig, you’d 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 don’t necessarily need a pressure gauge to do this; just listen to the check valve. When it stops chattering, you’re probably at the right point.
You can’t 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 shouldn’t use your service valve to throttle because someone will invariably close it to service the pump and then reopen it to its full open position. That sets your check valve to chattering again.¹
That’s sound theory. Let’s look at the practical side. People unfamiliar with pumps tend to think impeller vanes scoop the water into the pump discharge and into the system. Actually, the impeller or pump wheel “slaps” the water, throwing it to the edge of the pump casing and out the discharge. That creates a low pressure zone in the impeller’s 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.
In steam systems, we also need to consider net positive suction head (NPSH), defined as the net positive pressure that causes a liquid to flow through the suction piping to a pump and enter the impeller’s eye. NPSH is simply the minimum suction required to prevent flashing of the fluid in the pump.
There are two values for NPSH.
• NPSHR (R for required), a function of the pump design, is how much NPSH is required to keep the fluid from flashing to steam inside the pump.
• NPSHA (A for available), is what’s available for the pump to use.
In vented condensate systems, the NPSHA value is a function of several things:
• Positive static head, or water column, of condensate above the pump’s impeller eye
• Vapor pressure of the fluid
• Minus the piping friction loss of the water column piping into the pump.
Remember that vapor pressure is also related to temperature. Typically, the vapor pressure of condensate decreases as the temperature goes up.
Totaling the positives and negatives reveals available NPSH, which must always exceed required NPSH. If it doesn’t, the water in the eye of the impeller will flash back to vapor. This is called cavitation and can damage the pump’s internal components.
Domestic Pump and Hoffman Specialty have always been conservative in our pump sizing. The typical pump provided on your product has pumping potential far greater than your requirements. Let’s look at a typical 609 PF pump curve.
A pump with a 3-11/16” impeller might typically be selected for the duty point of 15 gpm at 20 psi (i.e. 46 feet of head). Left uncontrolled, or as Dan Holohan puts it, “not reined in,” that pump has the potential to pump at a capacity of 32 gallons per minute but will generate only 31 or 32 feet of head, or 13 psi. If you’re 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 out more steam 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.
Put another way, picture a backyard garden hose. With no nozzle in place, the hose will pump 10 gallons a minute, but only your ankles get wet—not your lawn. Installing a nozzle essentially gives you a control valve (i.e. the plug cock or balancing valve). Tightening the nozzle sends less water through the hose but increases the pressure; you can now spray water across the yard to even the farthest corners of your garden.
In the same way, the plug cock or balancing valve increases pressure and decreases flow. The pump moves back on its operational curve, preventing deadheading against the boiler’s internal pressure or a pressure drop in the pipe between the pump and next piece of equipment. Equally important, the pump moves back on its NPSHR curve, eliminating the potential for cavitation.
Look again at the pump curve and NPSH table above. For a pump rated 15 gpm at 46 ft TDH, the NPSHR is approximately 1 ft. required. But if the condensate is at 212ºF, then by the chart there is 0 NPSHA. NPSHA must be greater than NPSHR to stop the water in the impeller from flashing to vapor. So, if your pump remains uncontrolled and the condensate is too hot, then using same 609 PF at 210ºF (which has 1.4 NPSHA) and allowing it to run out on its curve to 26 gpm (which is 2ft. NPSHR) will result in pump cavitation. Reining in the pump with the plug cock or balancing valve not only increases the potential discharge pressure, but also moves the pump back to a point on the NPSH curve where the available is greater than the required. Now water can reach the boiler or the next piece of equipment in your system, and your condensate pump performs as it should.
So the next time you’re questioned about the system piping, startups and the need for a plug cock in the system, you’ll know that the answer is yes, your customer needs it and now you know why.
From the instruction manual section about putting the unit into service:
Throttle plug cock in discharge line until pressure at pump (while pump is discharging) approaches pump rated pressure. Tighten plug nut to secure adjustment.
From the troubleshooting section of the instruction manual:
Condensate Pump Is Noisy
1. The pump is working against a lower pressure than designed for. While pump is discharging, adjust plug cock in discharge line until pressure at pump approaches pump rated pressure.
¹ The Lost Art of Steam Heating, Dan Holohan, 1992, pp. 171-2