Stal discharge check rattle mystery

[Howeirdhughs]

Hello fellow misfits, this is my first post and I must say: "it is a pleasure to find a forum like this", since there are so few of us in the world.
Here is my question/mystery: I have a room with 3 Mycom screws 1) 75 Hp, 1)160 Hp, and 1) 300Hp. This time of year we have minimal load so the 75 Hp runs almost exclusively. during the times it hits 100 percent load on the slide I get some vibration on the package that transmits through the piping as physical vibration to the other 2 units through the pipe rack. The motor bearings feel smooth and the screw compressor is quiet yet the discharge piping vibrates at the end of the package. I also see what looks like a pressure pulsation on the oil pump discharge line, but all is quiet at the pump. The oil pressure on the McCormack control panel is digital so it is steady.
It appears that this has been a problem for a long time. I have been at this plant a little over a year now and I already had to replace the discharge check valve on the 75 Hp unit. The old one had shaken itself to death internally. What sucked is that it the body was welded in place to the unit. So the 75 Hp in now quiet, but the discharge check on the 160 Hp rattles when the 160 is shut down. The way I see it is that the spring pressure inside the valve should seat it during shutdown so I am assuming that it is also worn out. It is a STAL brand valve with nomenclature DN80 stamped on the body.
Questions: What could cause this vibration?
and does anyone know if I can get internal parts for a STAL valve and where?
Much love to the world, Howie

[nh3wizard]

As far as the parts go for the check, any supply / parts company should be able to sell you the check, DN 80 sounds familiar so they should be easy to find.

On the vibration portion, can you describe your system a little better, single stage, 2 stage, what type of refrigerant, what the system is doing when the vibration starts, have vibration analysis been done on the compressors.

[NH3LVR]

I have to think about your vibration. I am sending you the name of a supplier that can locate the valve for you. They are available. I got one a couple of years ago.

[US Iceman]

My first impression with hearing of discharge check valves rattling is the when the compressor completely unloaded. Sometimes when this occurs the valve does not see enough gas velocity to keep it open, so it rattles.

I've seen this happen on recip. compressors also.

...the screw compressor is quiet yet the discharge piping vibrates at the end of the package.

Have you checked to see if the pipe hangers are tight?

I also see what looks like a pressure pulsation on the oil pump discharge line,...

This may be due to the rotors spinning in the compressor housing also. Each time they spin past the oil injection port I've seen some machines that do this.

Just out of curiosity, is the volume ratio appropriate for your operating conditions?

Vibration problems are a pain to solve...

BTW, do you think the other Mr. Hughes would mind how you used his name?

[IceMan_4000]

Vibration can be a slippery one.

Vibration at 100% can be improper VI setting causing over compression.

I have also seen things as simple as how is the unit mounted to the ground. Without enough mass a screw can vibrate to death. this unit is not on a upper floor or on a steel floor is it.

When was the last time someone did a laser alignment on the unit.

1. Check alignment of screw and motor misalignment can cause big vibrations
2. ensure VI is properly set this can cause damage to the screw beyond the vibrations you have noticed.
3. ensure the unit is grouted down and has alot of concrete mass under it.
4. Get a vibration analysis done us vib techs can help spot issues and help prolong life of bearings.

Good luck

[RANGER1]

I have seen similar on screw where oil pump pressure fluctuates/pulsates due to inadequate mass flow rate to keep check valve open .
Check valve needs to be sized for min/max flows so valve can open .
Stal stopped making stop valves etc years ago i thought .
We sucessfully use duo type wafer check valves (2 flaps spring loaded ) , using high finnish steel/steel seats .