Steam check valve hammering

TeeSee

I originally posted this on the Wall last week.

Need some advice regarding a two-pipe steam system in a residential two-story Colonial in New England.

Arrived at this home with the steam system hammering the house off its’ foundation. The homeowner could not give me an exact time this started happening, but it had been going on for years. Going through the system, I discovered steam and water pouring out of the steam condensate pump vent and the check valve slamming hard after the pump cycled. Second, the check valve was leaking causing the pump to cycle even when the boiler was cold.

I came back 21 days later to find the VXT I just reset had passed 389 gallons since my first visit! So, new F & T trap innards and a new Hoffman #8 cleared up the steam and water pouring out of the vent pipe. All new Hoffman #4s cleared up the humid basement. Cleaning the mud out of the horizontal pigtail explained why the boiler never cycled. Piped it vertically and now we are running at .5 to 1.5 psi. What a difference…the system is now quiet except for that evil check valve at the condensate pump. While it does not slam anymore, it does what I call hammer gently or flutter when the pump discharges.

The condensate pump has a 1-inch discharge to the Hartford Loop with a ¾-inch check valve right at the Loop. See attached picture. I thought I would move a new 1-inch check valve to the base of the pump to solve the hammering the check valve produces when the condensate is hot. The check valve does not hammer when the condensate is cold or warm….only 180+ hot. It only hammers for about 30 seconds to a minute after the pump sends water back to the Hartford Loop, then to the boiler. Even moving the check valve to the base of the pump still causes hammering.

I tried this first in an effort to keep costs down, but I figure I need to repipe the pump discharge line to the floor, then up to the Hartford Loop. Right now, as seen in the picture, it comes horizontally out of the Loop, then a few swing joints, over to the receiver, then down to the discharge on the pump. I know the water line is above the Loop and check valve so theoretically both are immersed in water and should keep the steam out of the discharge piping. Now I’m thinking I have flash steam occurring at the Loop because who ever piped the discharge did not go immediately to the floor with a close nipple and elbow at the Loop tee. This would explain the broken check valve. Unfortunately, I cannot prove if the old check valve hammered because it was broken. Also, the new check valve I installed back at the Hartford Loop has a 90-degree seat…the one I took out has a 45-degree seat. Does that matter?

I would appreciate advice from anyone who has run into a similar situation or if someone can confirm my suspicions regarding the horizontal mess of piping from the pump to the Hartford Loop. Being so close to the finish line is aggravating with just one more hurdle to go. Thank you for your help!

Jim_158

A few thoughts...

First, throttle down on the valve at the pump discharge. No guarantee but it may help. The evaporation rate is likely only a small fraction of the pump capacity.

Second, what is controlling the pump? It should be controlled by the water level controller on the boiler, not the float switch in the receiver.

If you are controlling the pump from the boiler, throttle back on the discharge valve for a longer on cycle, but not so long that the boiler cuts out on low water.

The receiver, or boiler feed tank, should have a minimum of 1 to 1.5 gallons of capacity per boiler HP (1HP=33,475BTUH) or enough reserve to feed the boiler until the condensate returns from the system. Too small and it will overflow when the condensate does return. Good luck!

Pumpguy

This is not

a boiler feed pump, but a simple condensate return pump operated start-stop by it's float switch.  The low water make up should be part of the boiler piping, and not at this condensate return pump.  Most likely the receiver doesn't have enough water storage to function as a boiler feed pump.  



I have never seen a slamming check valve problem on a condensate return pump, and these are almost always fitted with metal-to-metal bronze swing check valves.  There are soft seated check valves available, in both Teflon and synthetic rubber (Buna, Viton, EPR, etc.) and these may be a solution.  Another thing to consider is a shock arrester just down stream of the check valve.  This can be constructed using a 1" Tee, a 1" nipple about a foot or so long, and a cap.  Commercial shock arresters are also available.



I would be concerned about that condensate temperature.  IMO, 180 degrees F is too hot.  I would like to see something colder, say 140 or so.  The usual causes of hot condensate are operating at too high a steam pressure, leaking steam traps or other causes of steam entering the return lines, or insulated return lines. 

TeeSee

Condensate temperature

I should add the condensate from the returns gets up around 180 degrees when I continually run the system for over an hour. The temperature at the radiator inlets is somewhere around 214 degrees with the radiator steam trap outlet pipes in the range of 190 degrees. There is not alot of heating load because it is spring time. Return condensate at 180 degrees did not seem unusual. What I did notice is when that pump discharges, I can feel warm condensate return water from the receiver being pumped to the Hartford Loop. Once the pump ends its cycle that valve starts to hammer and the discharge pipe gets screaming hot. Seems the equalizer water is trying to push its way back to the pump as the check valve hammers. As I mentioned before, could it be just the angle of the check valve seat?....the old broken swing check was a 45 degree and the new swing check is 90 degrees.

TeeSee

Solution to hammering

Read from Dan Holohan's "steam basics" about flash steam at the Hartford Loop and the reason for using a close nipple at the elbow, down to the return, to prevent flash steam. I took this basic pipe rule, plus the mention of the check valve with the teflon seal from Pumpguy, and solved that hammering problem.

I located a 45 degree teflon check valve, ( my local supplier only carries teflon checks in 45 degree seats, and brass on brass for 90 degrees seats), added a union and ball valve for service. I kept the pump discharge piping as low as I could to ensure the check valve would always have a water seal on both sides. See photo. Ran the system for an hour and a half with no issues. Customer and I spoke the other night. He said the heat came on the other morning and it was so quiet, he didn't realize it was on till he felt the heat from the radiators. Best compliment ever! 

Hope this solution can help someone else who has a similiar problem. Thank you to all who chimed in to give me ideas on how to solve it!