Reciprocating compressor

The rod load is at a maximum when the outer end of the piston is compressing. At that point, you have full discharge pressure acting over the entire surface of the piston. At the same time, the other end of the piston is drawing in gas and thus has suction pressure acting over a slightly smaller area (piston area minus rod area). The other end of the rod is outside the piston and only sees atmospheric pressure. Calculate the force acting on each side of the piston based on this method. The net force is determined by subtracting the suction side force from the discharge side force. Then determine the stress on the rod by applying this net force over the cross-sectional area of the rod. If that stress is greater than the allowable stress for the rod, you have a problem. The allowable rod load should be provided by the manufacturer. It will depend on rod size and material. I am not at work, so I don’t have access to the numbers we use for maximum load or stress in a rod.

In many systems where we use reciprocating compressors, the discharge pressure is constant and we must be very careful that we never allow the suction pressure to become too low which would exceed our rod loading limit. It is good idea to have automated alarms to alert you if you are getting a high rod load.

The volume of gas present in the cylinder is not important. The pressure is the key.

The method I refer to does not take into account the pressure drop across the suction and discharge valves. I am assuming that the pressure drop is the same for both. If that is true, the affects cancel each other out, approximately. That is to say, the pressure in the outer end is slightly higher than discharge pressure and the pressure in the frame end is slightly lower than suction pressure. The net force is approximately the same.

I am sorry. There is an error in my previous post. The pressure drop across the suction and discharge valves does not cancel out. Instead, it adds to the net force and to the rod load. It would be more correct to say, that well designed valves should have very little pressure drop and should not cause a significantly greater rod load.

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Besides, as JJPellin explained, discharge gas temperatures are often used as alarms or cutouts to protect the rods rather than the lubricant, since high temperatures may indirectly serve to indicate high pressure ratios with possible overstressing of the rod.

Rod Load can be misleading on some compressors. Some compressors fail in the crosshead pin as the weakest place, others in the connecting rod or bearings. I was saw a mwtalurgical engineer ignor this and he replaced the standard rod with one made of a higher tensil strenght material. The unit was back in the shop next month with the side of the compressor brken out when the crosshead pin failed and the crosshead whent through the distance piece and broke the frame.

Liquid in a cylinder and pressure! The pressure will go to infinty because the liquid won't pass through the valves quick enongh and it won't compress. You will se broken, bent, distroyed rod, crossheads... I even saw a fly wheel weld it itself to the crank when the cylinder was had some water in it and it stopped the compressor cold.

There is some irreducible clearance in the cylinder. If the amount of incompressible liquid is less than that volume, then not much happens. A tiny bit more liquid can be taken up in flex in the rods and valves. A tiny bit more than that will always break something. Generally you break a valve, sometimes a rod, sometimes it just stops the machine and takes out something unexpected (like dcasto's fly wheel example), and sometimes it sends pieces of steel all over the landscape.

For an 8-inch cylinder with 3.5 inches of stroke and 6% built-in clearance, you can ingest 5.84 fluid oz (not quite 2/3 cup, 173 mL) without a problem, but 5.9 oz would probably break something.

David