Understanding How Condensate Pumps Do What They Do

Written on: August 9, 2021 by George Carey

As I am writing this column, the summer of 2021 is getting underway. It seems like a strange time to talk about steam systems and condensate handling equipment, but as you all know, the heating season will be here before we know it, and steam systems will be turning on as the cool nights eventually arrive. Condensate pumps, when used in steam systems, play an important part in the proper operation of that system.
Understanding the operation of these pumps is pretty straightforward. The condensate pump is made up of a pump and motor with an impeller on the end of it, and a receiver that the pump and motor are mounted on (usually cast iron, but steel can also be used). Cast iron receivers are more rugged and last longer in corrosive environments; condensate usually has a low pH (which makes it acidic). Inside the receiver is a float assembly (not unlike a ballcock found in a toilet tank) that is connected to a floatoperated electrical switch, which is attached to the receiver. The receiver has an inlet opening near the top side of the receiver; it also has an opening on the top where a vent pipe is connected.
The condensate return piping from the system is connected to the inlet connection of the receiver; the vent pipe is used to vent the air from the system. As condensate and air travel along the return piping, they enter into the receiver through the inlet connection. The condensate falls to the bottom of the receiver while the air enters into the receiver and exits out through the vent pipe located at the top of the receiver.
Therefore, in effect, the vent pipe of the condensate pump is the main air vent for the system. Make sure that there are no water pockets in the return piping because that could prevent air from getting out of the system. Also, never plug the vent line because condensate pumps are not rated for pressure. If they become “pressurized” because of a blocked vent line, they could explode!
As the condensate continues to gather in the receiver, the float part of the assembly starts to “float” up/rise up with the rising level of condensate. At some point, the float-operated electrical switch “makes”—closing a set of contacts that turns the pump on. The condensate pump discharges the condensate to either the boiler directly or possibly to a boiler feed tank in the boiler room. Naturally, as the pump discharges the condensate from the receiver, the water level, as well as the float in the receiver lowers to a point where the switch “breaks.” This opens the contacts that turn the condensate pump off. This is basically what a condensate pump does—as returning condensate enters the receiver, it raises the float, which eventually turns the pump on. As the water level drops in the receiver, so does the float—eventually turning the pump off.
Important details
There are a couple of details that you need to pay attention to when piping a condensate pump into the system. On the inlet side of the receiver, it is good piping practice to install some type of strainer (a Y-strainer or basket strainer are two popular choices) just before the condensate enters into the receiver. There is sediment in old steam systems that the condensate will pick up as it flows back to the receiver. If the sediment makes its way onto the face of the pump’s seal, it may groove lines into the carbon and ceramic seal, causing it to leak. If the leak isn’t discovered quickly, the condensate will work its way into the bearings of the motor, causing it to eventually fail.
On the discharge side of the pump, there should be a check valve, a balancing valve and a service valve. The purpose of the service valve is to isolate the pump from the system; the service valve can be closed if you need to work on the pump. The check valve is needed so that whenever the pump is off, the water that is in the piping on the discharge side of the pump doesn’t fall back into the receiver. If the check wasn’t there, or if sediment gets underneath the flapper of the check, the water would flow back into the receiver causing the float to rise and bring the pump on. The pump would unload the receiver and shut off, and this cycle would “seesaw” back and forth endlessly.
If you happen to walk into a boiler room in the summertime and the steam boiler is off because it is used for heating, yet the condensate pump turns on and then turns off…and then turns on, and then turns off again…most likely the check valve has been compromised. Probably dirt or sediment is keeping the flapper from seating properly on its seat.
The balancing valve is used to provide a certain amount of “back pressure” on the pump. Standard stocked condensate pumps are rated to pump the condensate at a discharge pressure of 20 pound-force per square inch gauge (psig). However, in most applications, the boilers are running at very low pressure, typically 2–5 psig. When the pump turns on and it doesn’t “see” 20psig of pressure, it will run way out on its curve, pumping too much condensate, too fast. It can cause the check valve to chatter, creating unnecessary noise. By closing the balancing valve, you are creating the additional pressure the pump was designed to work against, thus slowing the flow rate down to where it can operate properly and quietly.
Pump problems
I get calls every now and then complaining that the condensate pump is turning on, but the pump isn’t pumping the water out of the receiver. In some cases, the water starts pouring out of the vent pipe. Usually the problem is related to the temperature of the condensate. I am not talking about the temperature that exceeds the material of construction or the pump’s seal rating (most are rated at 250°F). What I am referring to is when steam is allowed to enter into the return lines (usually bad traps that have failed in the open position) and elevate the returning condensate’s temperature above 185–190°F.
When the condensate becomes too hot and gets close to its boiling point, there isn’t enough pressure on the water to remain a liquid. When the pump turns on and the water enters into the eye of the impeller, it experiences a drop in pressure. Because the water is so close to its vapor pressure (boiling point), it flashes into vapor/steam. Of course, the impeller isn’t capable of pumping steam, so the impeller is spinning at 3,400 rotations per minute (rpm), and no water is discharging out of the receiver. This isn’t because of a bad pump; the water is simply too hot. There isn’t enough pressure on the water to keep it in its liquid state.
Remember—this is an open system and the only available pressure is the height of the condensate that is sitting in the receiver, which is less than a foot. In a closed hot water heating system, you have a fill valve and an expansion tank that provide a lot of pressure, so the pumps have no problem circulating the 200°F water.
The answer to this problem is to lower the temperature of the condensate, so fix the radiator traps that are leaking and make sure the pressuretrol isn’t set too high. There is a relationship between the pressure of the steam and its corresponding temperature, which holds true to the temperature of the condensate. This is another reason why there is no benefit to cranking up the pressuretrol setting in a heating application. If you have any questions or comments, e-mail me at gcarey@fiainc.com, call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM