A very scary service call.

RayWohlfarth

We just started servicing the boilers for a local customer. The building was a three story school. When I arrived, the maintenance person told me his pipes sometimes shake and he wanted to know what I thought. I told him we would check. Two other companies had been there this past heating season. Each boiler had an isolation valve and I thought perhaps the valves would shut while the pump was running. When one of the boiler started, the pipes started to rattle and sway. We shut off the boilers and looked around. The PTA gauge was on top of the boiler and could not be seen from the floor. I found two gauges on the pipe and they read 2 Psig. I took a deep breath. We let the water cool and added more water and the shaking stopped. It was a scary service call.
I was asked what was causing the shaking and swaying and I was not sure. Your opinion would be appreciated.

Jamie Hall

This is hot water, Ray? 2 psi and a three story building? I'd wonder if the pumps weren't either cavitating badly or losing flow entirely, then picking up a slug of water and whanging it into the pipirng. Rinse and repeat...

Zman

I like Jamie's pump theory. I could also have been microflashes of steam in the boiler due to the low pressure. Inconsistencies in the scaling in old boilers can cause hot spots leading to violent shaking.

RayWohlfarth

Jamie and Zman Thanks for the expertise I never saw anything like it. The maintenance person was nonchalant about it. Have a great weekend.

Solid_Fuel_Man

Steam or water?

EBEBRATT-Ed

Was the water temp high enough to make steam? Not likely. Probably cavitation due to the low water pressure at the pump inlet. Could have had steam at the pump inlet even with lower water temperature as pump drops the pressure at the inlet

Zman

I have had a similar rumble on and old cast iron hulk . Boiler would run perfect at low fire and shake the room when it went to high. Was to scared to descale in the winter, figured the scale might be holding it together.

RayWohlfarth

Solid Fuel man It was hydronic
Ed It was 180 degree water

RayWohlfarth

Zman You are probaly right

Steve_210

Dry fire
Obviously there’s no low-water cut off
I would not turn it on again without ensuring they install one

Steve_210

There should also be a manual reset high limit on each bOiler as well as a low water cut off.
If you’re the last one there and something happens when you leave you could get Into Trouble

SuperTech

I second the motion to add a low water cutoff to that boiler. It's code in my area.

DanHolohan

@RayWohlfarth what size were the pumps and where were they in relation to the compression tank?

RayWohlfarth

Steve and Suopertech Each boiler has a low water cutoff.
Dan The boilers were up stream of the circulators and the circulators were pumping away from the expansion tank. The system was not piped primary secondary. They used isolation valves and speed controls on each boiler. I do not like isolation valves for boilers I initially thought the isolation valves were closed and dead heading the pump. The pipes only started dancing when the boiler furthest from the pumps started. Weird.
I was thinking of getting a Rabbi and a Priest

DanHolohan

@RayWohlfarth when they ran the far boiler, did they isolate the other boiler, or were they both running at the same time? Are there check valves between the boilers to prevent short-cycling flow back to the pump suction?

ScottSecor

I agree, at 2psi there could be some cavitation going on. Something to think about is NPSH (I'll let Dan explain). Also, there appears to be a snubber on the bottom of the pressure gauge, I have seen them clog and basically deem the pressure gauge worthless. Not to sound like safety is not a concern, but a low water cutoff or a manual reset aquastat would not fix the vibrating problem (yes they are required).

With regard to Rabbi and Priest, you have to make a decision because you don't want to offend.

RayWohlfarth

Dan There are check valves on each boiler. I am not sure whether they isolate the lag boiler. I would assume so. This was our first visit there. Most good control companies would use an end switch to verify the valve was fully open prior to firing. I will let you know when we return.
Scott, Thanks thats a good point. I looked at three gauges and they all read the same pressure. That is why I love this industry; I learn something new almost everyday
I am hoping the added water will keep the religous people away. LOL
Thanks

DanHolohan

@RayWohlfarth @ScottSecor Guys, I'm focusing on the fact that the violence stopped when Ray raised the pressure. The pumps are pumping away from the compression tank, which is the point of no pressure change. There will be a positive pressure at their discharge and a very low pressure at their suction if the system pressure is low. That will create low NPSH as the pump suction and serious cavitation. Cavitation doesn't turn the water to steam because we're not added any heat to the water. It just flashes the water to vapor, which then collapses at the edge of the pump's impeller with micro-water hammer that will destroy the pump. If there's short-cycle pumping that water hammer might be transmitting back through the pipes. I'd make sure those check valves are working properly.

RayWohlfarth

Dan Thanks for the expertise. I will repost once we return to the school

DanHolohan

Thanks!

Solid_Fuel_Man

I would also like to verify there is actually water flowing through the last boiler make sure there isn't a stem on one of those valves broken.

RayWohlfarth

Solid Fuel Man, Thanks I will check that once we go back