Say “no” to suction strainers for condensate return and boiler feed units
Volume 5/ Issue 2/ August 2018
Condensate return and boiler feed unit specifications frequently call for a strainer to catch particles that could damage pumps. Basket strainers or “Y” strainers are equally effective, as long as they’re properly located and maintained.
Suction strainers, on the other hand, aren’t a good idea. They made sense in the 1920s and 30s, when the only pumps available to provide high pressure returns were turbines—whose close tolerance made fitted suction strainers a good choice to stop particle damage. But by the mid-1950s, several companies now within Xylem developed single- and multi-stage centrifugal pumps that could deliver higher pressures. These pumps have broader tolerances; solids up to 1/8” diameter can pass through the impeller of many Bell & Gossett pumps with no real harm.
Specifying engineers started to recommend the centrifugal design and multi-stage pumps for condensate return and boiler feed units. But old habits die hard; the engineers kept on specifying suction strainers. Let’s look at why that’s a problem – and how you can fix it.
The condensate unit is a tank, a pump, and float control at its simplest. If you put a suction strainer between the tank and the pump, the tank will fill, and the float control will turn the pump on. But if such a strainer is not maintained, it restricts water entering the pump. That starves the pump and maybe even runs it dry, damaging the seal.
You’ve also added a pressure drop between the tank and pump, across the strainer, reducing NPSHA to the pump. Now the pump’s NPSHR may exceed NPSHA—and the pump cavitates. So even if it doesn’t run dry, the pump is still damaged and the seal destroyed. This is especially likely when units are vented to atmosphere, and you’re dealing with hot condensate.
Plus, if the strainer hasn’t been maintained, it’s a good bet the system traps haven’t been maintained either. When they start to fail, the condensate returns even hotter, with even less NPSHA due to its temperature.
What’s the solution? Use a strainer, but move it to the inlet of the tank. The strainer still catches particles before they get to the pump, but now it also reduces sediment in the tank. If the strainer gets plugged, water doesn’t get into the tank, the float switch doesn’t rise to start the pump, and you don’t run the pump dry. If not enough condensate returns, a small system’s boiler shuts down, and a large system increases make up water into the feed unit—both signaling the need for maintenance.
In moving the strainer to the unit’s inlet, you may need to change the strainer type—especially where space is at a premium. A “Y” strainer may require back pullout space to remove and clean the strainer screen. Basket strainers don’t need back pullout space. Usually you can remove the lid or cover plate and lift the strainer screen out vertically. The strainer screen is somewhat self-cleaning, and the basket strainer usually has a large dirt pocket to collect system debris—especially useful for slower gravity returns. The bucket in which the strainer sits typically has a drain at the bottom. Just remove this and hose out system debris from the basket strainer body.
Larger systems with multiple zones might have a mix of pumped returns and gravity returns to the feed unit. If the designer has done the job right, pumped returns are sized for the pressure drop between the condensate and feed unit plus 5 to 10 psi, depending on the pressure drop. Flows entering the feed unit’s strainer at 5 to 10 psi will not create problems for the basket strainer. Remember, plugged strainers at the feed unit inlet will probably cause more make up water to be added to the feed unit or directly to the boiler. If you see an increase in use of make up water or treatment chemicals, it’s time for a system review. Higher energy bills are another indicator; make up water is colder and needs more energy to make it back to steam, increasing the bill to run the boiler. These indicators are all easy to monitor—and the solution is so simple. Clean the strainer.
An old joke about how specifications live forever starts by comparing the distance between train rails and going backward in time. The specification for the suction strainer is just such a case. In today’s steam systems with centrifugal style pumps, the answer is inlet basket strainers. If you’re writing the spec, update it. If you’re bidding on a spec that hasn’t been updated, take exception and quote the basket strainer—and include this article with your response. Giving specifying engineers this knowledge gives them the power to save money for everyone – including your customer.