In this article, we learn about the Reciprocating Compressor Piston Rod Load – Principle and Calculation method. The compressor rod load exceed the recommended rod load (ie., Combined Rod Load) of a reciprocating compressor leads to major compressor failure. For this reason, it is important to understand the rod load during the design stage and startup of the compressor.
Loads Act on Piston
First, we will see the loads acting on the piston rod. The following three types of loads are act on the piston.
- Gas Load
- Inertia Force
- Combined Rod load
Gas Load:
As the compressor piston moves to compress the gas, the differential pressures acting on the piston and stationary components result in gas forces. This force is called Gas Load, referred in API-618 (paragraph 6.6.2).
Normally the gas load act on the piston rod is two types. They are
- Gas Load Under Compression
- Gas Load Under Tension
The Gas Load Under Compression
Consider a double-acting cylinder in that the piston moves toward the head end refer below figure. The discharge pressure (Pd) act on the piston tends to compress and buckle the piston rod. At the same time, gas is entered into the cylinder behind the piston (Frame end), exerting suction pressure force(Ps) on the backside of the piston. Two forces are opposite in direction, but since the discharge pressure is greater, the net resultant force compressing the piston rod called Gas Load Under Compression.
The formula used to calculate the rod load
Fg = (PdxAhe)-(PsxAce)
Where
Ps = suction pressure, psia
Pd= discharge pressure, psia
Ahe= cylinder head end area, sq. in.
Ace = cylinder crank end area, sq. in. = Ahe- rod area
Gas Load UnderTension
Now let us consider the piston moves toward the frame end on the return stroke, as seen in below figure. The net force of suction and discharge pressure creates a tension load on the piston rod. This is called Gas Load Under Tension.
Fg = (PsxAhe)-(PdxAce)
Where
Ps = suction pressure, psia
Pd= discharge pressure, psia
Ahe= cylinder head end area, sq. in.
Ace = cylinder crank end area, sq. in. = Ahe- rod area
Inertia Force
Inertia force is the force resulting from the acceleration of the reciprocating mass. Usually, the inertia force referred with reference to crosshead pin.
It is the summation of the products of all reciprocating masses (piston and rod assembly, and crosshead assembly including pin) and their respective acceleration.
Combined Rod Load
It’s the algebraic sum of Gas load and Inertia force act on the crosshead pin.
Fcombinedrod load=Fgas load + Finertia
As per API 618(6.6.3), The combined rod loads and the gas loads shall be calculated for each 5-degree interval of one crankshaft revolution. The point at with the combined rod load change it magnitude from Tension to Compression (negative to positive sign) or vice-versa is called as “Load Reversal”. This Load reversal act on the crosshead pin during each revolution.
During the design stage of reciprocating compressor, the manufacturer makes sure the duration and magnitude of load reversal shall consistent with the design of oil distribution of crosshead bushing in order to maintain proper lubrication and to avoid crosshead pin failure.
Reciprocating Compressor Piston Rod Load Calculation method
For calculation of piston rod load consist of three steps.
Step 1:
First, we need to find the gas load act on the piston. As mentioned using the above two equations, the gas load act on the piston calculated for every 5oof one complete crank revolution. The Blue colour line represents the gas load act on the piston. Refer graph below
Step 2:
The inertia force act on the piston due to the all reciprocating masses. Refer graph below, the gas load is also calculated for every 5oof one crank revolution.
Step 3:
The algebraic sum of Gas load and Inertia force act on the crosshead pin. Refer graph, the red line indicates the combined rod load for every 5o of one crankshaft revolution(ie., 0 to 360o).
In the graph, the black dots at the crank angle of 92o and 320o represents the degree of “Load reversal”.
As per API 618 (6.6.1), In any case, the combined rod load shall not be higher than the Maximum allowable rod load. In our example the maximum allowable rod load is 300kN. As we have seen from the figure the combined rod load doesn’t reach the maximum allowable rod load during one complete revolution.
In some case, the compressor shall be capable of handling short duration excursions of operation involving a load increase up to 10% above the maximum allowable continuous combined rod load and/or maximum allowable continuous gas load. These excursions shall be limited to a duration of less than 30 seconds and a frequency of no more than twice in a given 24-hour period API (6.6.5).
Conclusion:
Both the Compressor manufacture and the Purchaser should ensure the Rod load maximum value to know the reciprocating compressor limits. Also, the rod load curves and peak rod load/reversal values provide an important insight to predict the health of a reciprocating compressor.