Rod Load Tension and Compression

[pcog07]

Hello, would anyone be able to explain what are the variables that causes a "rod load tension/compression" to rise on a recip (process) compressor and what can you do to lower it? Thanks in advance.

 

[zdas04]

Sure. A lot of people here can explain this.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[BigInch]

Basically maximum rod load is discharge pressure x face area of the piston.
Reduce the discharge pressure to lower the rod load.

Independent events are seldomly independent.

 

[zdas04]

For a double acting cylinder, the controlling parameter is compression ratios, not raw discharge pressure (because even at max discharge pressure in one compression chamber you have suction pressure pushing the piston in the opposite direction relieving some of the load).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[pcog07]

Thanks for the info. Have a great day.

 

[Aljabobi]

 

[dcasto]

actually,its not ratios, its straight out differential pressure Here's a rule of thumb, look at a cylinders maximum working pressure. Dive that by 4. That is the minimum suction pressure at the maximum working pressure where max rodload will occur. It's an observation.

 

[pcog07]

@ dcasto: "dividing the max working pressure by 4". From where was this constant "4" derived from? Thanks for the info.

 

[zdas04]

Sorry, but when you dig into the math, it is ratios not differential pressure. I can get the same rod load with any discharge pressure but the same compression ratio. And maximum rod load does not require maximum discharge pressure. I've seen rods fail at less than 50% of MAWP, maximum pressure is just not a factor.

The "4" is a rule of thumb for maximum compression ratios in a non-cooled conventional cylinder. We use it for design conditions, and it is really conservative. I've seen recips run for years with 5 ratios. I've seen recips fail with 4.2 ratios. When I'm picking a frame for an application I use 4 ratios/stage as my design point. This means that if the discharge pressure divided by the suction pressure is > 4 I'm looking at a 2-stage, > 16 I'm thinking 3 stage, > 64 is probably a 4 stage, etc. It is conservative, but being conservative is a pretty good way to avoid broken.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[dcasto]

so a compressor with a 2000 psi discharge pressure and 4 ratios can withstand 1500 psi of differential with a 1" diameter Rod? I don't think so, a 1" Rod has a limit of 5000 pounds force.
Now if the piston was 2" diameter you could get a 1500 psi differential.

The easiest way to estimate compressor cylinder sizing is maximum rod load divided by area of the piston, that gives you differential pressure, from there you can get ratios given either suction or discharge pressure. BUT, ratios are a unknown until the very end, not in the beginning.