Recip Compressor Shutting Down Loaded

I am unclear whether this is two compressors in parallel or in series, as the wording "two stages" seems to imply.

I don't think the loaded shutdown would be harmful other than some roughness in pressure control. This would happen on power failure, for instance. On start or restart, the controls should ensure the compressors are unloaded. If this is a two-stage package, the high stage should start first, reducing the intermediate pressure to avoid stalling the low-stage compressor.

Sorry for the confusion, it is indeed two separate, multiple stage machines in parallel. Yes, on restart, the machines will be unloaded for sure.

As you have said, it is fairly common practice to unload recips before stopping. However, I cannot think of any adverse effect on the compressor if it is shut down loaded. It will decelerate much more rapidly than if unloaded and may put some braking load on the coupling and driver, but I cannot imagine that this would pose too much of a problem.

On all of the reciprocating compressor units I've worked on (Refineries), the emergency shutdown (trip) will kill power to the driver while simultaneously load the compressor to 100%. Reasoning is that under emergency (trip) conditions the compressor must come to a full stop to prevent injury or (further) damage (loss of oil pressure, low suction pressure, high discharge pressure, high vibration or extreme rod drop readings). In case of a normal (programmed) stop condition, the logic controller will usually follow the reverse of the startup loading sequence; e.g. Unload to 0% in a programmed time frame or steps, then stop the driver followed by a post-lubrication cooldown period. I hope this helps you.

Stopping suddenly under full load in some situations like power failure etc is nothing abnormal for the compressor. In fact, it is taken care while designing by OEM. Please get your doubt clarified by them. You have not indicated the working fluid.

Look for faulty unloading mechanism/ solenoid or even relay signalling impending (normal) shut down.

Thanks everyone for the responses, it seems to be that you would normally program the machine to unload and then stop but there are some scenarios (such as the E-stop that one poster mentioned) where it may not happen.

I have worked with 2-stage refrigeration compressors for many years (some paired in parallel and others single use) and note that no one has mentioned the risk of compressor damage when starting under full load. I believe there is a risk of bending a connecting rod or stalling/overloading the motor. This is most likely if liquid refrigerant gets back to the compressor but I think it is also possible if handling huge volumes of gas through valves that generally handle much lower volumes. It will all depend on the situation and the compressor. The safety cut-outs etcetera try to prevent this but there is the odd time when they will turn out to be inadequate.

The usual time that we have to start under full load is after having to turn off the power for some time, so that the pressure equalises around the system and these days you cannot just vent off some pressure to atmosphere just because there is no frig. engineer handy! Normally we do get away with a start under equalised pressures but we always watch like hawks for the protection devices to work properly and no nasty noises to happen.

Hope it's useful.

I think the poster has no such problem which you have encountered perticularly for 2-stage refrigeration compressors. He has not even specified the medium. Please check whether your evaporator is above the compressor which is route cause for liquid entry in to compressor. Starting with load is certainly to be avoided. But stopping with load will not cause any adverse impact on the machine.