piston rod reversal

[waqasmalik]

Hi every one, i am new to reciprocating compressors and i want to know about piston rod reversals.Can anyone please tell me in detail what is this phenomenon? Is it self created or being created?Please help me

 

[waqasmalik]

someone reply plz

 

[zdas04]

It is Saturday in the US, this site is always slow on Saturday and Sunday. Sometimes free advice takes longer than 11 hours.

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"

 

[waqasmalik]

lolz, ok, me waiting

 

[ivymike]

I suppose the short answer is that the compressors are called "reciprocating" because the pistons go back and forth. When the piston & piston rod change directions, they call that a reversal.

Similar concept in gasoline / diesel recip engines - "Upper Top Ring Reversal Point (UTRRP)" is where the top ring stops moving up and starts moving down.

 

[Montemayor]

waqasmalik

If you are “new” to reciprocating compressors and need to know about them, then you should purchase a copy of the GPSA Engineering Databook and read Chapter 13, Compressors and Expanders – from whence I quote:

Rod Loading
Each compressor frame has definite limitations as to maximum speed and load-carrying capacity. The load-carrying capacity of a compressor frame involves two primary considerations: horsepower and rod loading.

The horsepower rating of a compressor frame is the measure of the ability of the supporting structure and crankshaft to withstand torque (turning force) and the ability of the bearings to dissipate frictional heat. Rod loads are established to limit the static and inertial loads on the crankshaft, connecting rod, frame, piston rod, bolting, and projected bearing surfaces.

Good design dictates a reversal of rod loading during each stroke. Non-reversal of the loading results in failure to allow bearing surfaces to part and permit entrance of sufficient lubricant. The result will be premature bearing wear or failure.

 

[dcasto]

clipped from ariel's tech library

Minimum Allowable Pin Load Reversal

Crosshead pin reversal is a reversal of compression and tension loads at the crosshead pin to connecting rod bushing. Without proper reversing loads, this bushing will not be provided with sufficient lubrication and bushing failure will occur. Pin reversal is defined by two components, degrees and percent. These represent the duration of the reversal and the magnitude of the reversal. Both of these values must meet or exceed minimum values. Ariel's requirements for reversal are 30 degrees of crank rotation, and 25% magnitude. The percent magnitude is defined by the smaller of the tension or compression force divided by the larger of the two. The combined gas plus inertia loading at the crosshead pin is used for the reversal calculations. The inertia load component includes the weights of the piston and rod assembly, balance nut and crosshead with crosshead bushings.

Ariel's Reciprocating Compressor Performance Program provides the reversal values and will flag upon insufficient reversal. A review for reversal will need to include the range of speed and any single acting conditions. Tandem cylinders may present sufficient reversal at the maximum and minimum speed, while insufficient reversal somewhere in between. The multi-run function proves to be an invaluable tool in reviewing reversal values.

Some smaller SG style tandem cylinders are rated for lower reversal values, 30 degrees and 15%.