By Dave Ruckman
How is the heating hot water (HHW) pump working in your building? Do the seals seem to wear out far too quickly? Does it always look black when you open it, as though it is burnt on the inside? I have seen these symptoms on a number of occasions—usually in older buildings. What typically happens is that the same parts are put back into the pump, and six months later the seal is leaking again. Our objective at IPEG is to always increase the mean time between failures of your pump, particularly if we are aware there is an ongoing problem. We will look at options, materials of construction, type of pump and other possible factors.
The first thing to do is assess the problem before it is possible to find a solution. If there is premature seal failure, there are always several questions to answer:
- Is there excessive vibration when the pump runs?
- What is the temperature of the water being pumped?
- Is there adequate water supply to the inlet of the pump?
- Is the pump running on the curve?
- If the pump is black internally: Why is the pump black internally?
Excessive vibration in a pump will cause premature failure in the seals. You don’t have to have expensive test equipment to assess excessive vibration in the pump. If you can feel a lot of vibration when you place a hand on the running pump, it most likely has excessive vibration. It can be caused by a bad pump bearing, motor bearing, a worn coupling, an unbalanced impeller, closed inlet or discharge valve, pipe strain, poor installation or many other things. The cause of the excessive vibration must be corrected in order to increase the seal life.
Temperature of Pumped Water
If the pumped water temperature exceeds the limits of the elastomer used in the seal of the pump, this may have caused the leak in your seals. For most HHW applications, this will not be a problem as the high temperature range is normally within range of a standard Buna mechanical seal. It may be good to eliminate the elastomer temperature limit as a problem if the pumped water temperature is above 270 F.
Water Supply to the Pump Inlet
It is always a good idea to have a working pressure gauge on the suction and discharge of a pump to help evaluate problems. The suction pressure required by the pump (NPSHr) is unique to each pump model and can be determined on the performance curve of most pumps. The pump will cavitate and will perform poorly if the available pump inlet pressure (NPSHa) runs lower than the NPSHr on the performance curve. Cavitation can cause many different problems in a pump. Early seal failure is one of the results.
Running on the Curve
HHW pumps, like all pumps, are often oversized for the application. If the pump is oversized, it may be running below the minimum continuous duty point of the pump. Dead head is included in this category. Pump performance below this point can be very unstable and create vibration, cavitation, high bearing loads, high temperatures due to inadequate circulation and seal failure. This point can often be found on the performance curve, or you may have to consult the pump manufacturer.
It also creates instability in the pump if it is undersized and running on the far right side of the performance curve near runout. Artificial head pressure can be created with a valve on the discharge side of the pump to get the pump running in a more stable place on the performance curve. It would then have to be determined if there is still adequate flow for the application.
Pump is Black Internally
I have seen this many times. The problem each time was caused by a high concentration of iron in the pumped water. I was able to come to this conclusion by scraping the black off of the seal and inside of the casing, and I then held a magnet to the scrapings. If the magnet picks up the scrapings it is a pretty good indication that there are iron fines in the water being pumped.
The iron fines can be removed from the pumped water by using a filter, magnetic strainer basket or other similar means. If controlling the iron fines is not an option, it is possible to put a hardened seal in the pump. Most HHW pumps are originally specified with a standard Buna or EPDM mechanical seal with carbon graphite and ceramic seal faces. Typically the carbon graphite seal face has been completely worn down to the metal seal holder by the iron fines. With most pumps there is usually an option to get hardened seal faces. Tungsten carbide with carbon graphite faces are commonly offered, but I have not found them to be effective. The carbon face still wears down. Silicon carbide seal faces have worked very well. I have been very successful finding silicon carbide seal faces in the aftermarket. They are often at a lower cost than the standard seal from the OEM.
Another way to attack the premature seal failure due to iron fines in the water is to replace the pump with a Grundfos wet-rotor design circulating pump, which does not need a mechanical seal. The only seal required on the wet-rotor pump is a case O-ring.
If you have continuing problems with mechanical seals in your HHW, or any other pump, please contact your IPEG representative for an evaluation of your current pump or for help finding a replacement pump.