Do I need a high temperature seal for my condensate pump?
Volume 3/ Issue 2/ May 2016
Rapid or repeating pump failures often lead clients to ask if a high temperature seal is needed. But it’s more likely there are NPSH problems.
Keep in mind that the maximum temperature that can be reached in the receiver is 212ºF. Domestic Pump condensate and boiler feed receivers are vented to atmosphere, and anything higher than 212ºF is steam. Seals are rated for 250ºF water with a pH range of 7 to 9. So the temperature in which the seal is used can’t exceed the temperature limitation of the original material.
Here’s the real story. When condensate or feed water is returned at 212º F, there is 0 ft. of NPSHA in the water. The pump starts, the impeller in the pump volute creates its low pressure zone—and NPSHR exceeds NPSHA, causing the water inside the pump to flash to steam. The resulting cavitation mimics running the seal dry.
The failure is caused by heat and friction. Temperature affects both the chemical and physical properties of water. For example, the viscosity of water drops rapidly as temperatures rise. Water at 70ºF is 1.0 centipoise; at 212ºF, it drops to 0.3 centipoise and loses its lubricating properties. Reduced lubricity between the seal faces can increase seal face wear. Thus, elevated temperatures make the secondary seal, or O-rings made of Buna-N, susceptible to damage, and the seal eventually fails.
Pumps are selected for specific duty points, based on the NPSH characteristics of the pump and the design temperature for the complete pumping package. For example, Domestic CC unit literature notes maximum operating temperature of 200ºF. You can infer that the pumps selected require less than 7.5 feet of NPSHR, because the water at 200ºF has 7.5 feet of NPSH available at 0 feet of elevation above the pump suction.
When available NPSH is greater than required, there’s no cavitation, and the pump may provide years of service without seal failures. But when steam traps fail and condensate is returned too hot, or when the product is improperly selected or installed, cavitation may occur. Installing a high temperature seal won’t cure the problem; in fact, this much more expensive seal will also fail quickly, because no seal is designed to run indefinitely in a pump that is cavitating.
For 200ºF condensate or below, standard Centriflo condensate pumps are appropriate. For temperatures above 200ºF, you must use the Domestic “B Series, 2ft NPSH pumps,” which have low NPSHA values at these temperatures.
In short, high temperature seals are not typically required for condensate or boiler feed applications in steam systems. Failing seals are often symptoms not of a problem with the seal, but of a problem with the system. Treating the symptom without addressing the cause can be expensive and ineffective.
For installation issues, see the November 2014 SteamTeam newsletter on start-up pump balancing. Click here