Principle And The Calculation Method Of Reciprocation Compressor Piston Rod Load
Most major harm to the compressors is caused by rod load. If the rod load surpassing the suggested rod load (ie., Combined Rod Load) of a reciprocal mechanical device. For this reason, it’s necessary that the operators and engineers should perceive the rod hundreds whereas choosing the reciprocal mechanical device or throughout startup of the mechanical device (Compressor)
First, we are going to see the masses engaged on the piston rods. the subsequent 3 styles of masses ar engaged on the piston.
- Gas Load
- Inertia Force
- Combined Rod load
Gas Load
As the mechanical device piston moves to compress gas, the differential pressures engaged on the piston and stationary parts end in gas forces. This force is named Gas Load, referred in API-618
Normally the gas load act on the connecting rod is 2 sorts. They are
- Gas Load underneath Compression
- Gas Load underneath Tension
Gas Load underneath Compression
Consider a double acting cylinder therein the piston moves toward the top finish refer below figure. The discharge pressure (Pd) act on the piston tends to compress and buckle the connecting rod. At constant time, gas is enter into the cylinder behind the piston (Frame end), exerting suction pressure force(Ps) on the rear aspect of the piston.
Fg = (PdxAhe)-(PsxAce)
Where
Ps = suction pressure, psia
Pd= discharge pressure, psia
Ahe= cylinder head finish area, sq. in.
Ace = cylinder crank finish area, sq. in. = Ahe- rod space
Gas Load UnderTension
Now allow us to contemplate the piston moves toward the frame endways the come back stroke, as seen in below figure. information superhighway force of the suction and discharge pressures leads to a tension load on the connecting rod. this is often known as Gas Load underneath Tension.
Reciprocating mechanical device connecting rod Load – Principle and Calculation methodology
Fg = (PsxAhe)-(PdxAce)
Where
Ps = suction pressure, psia
Pd= discharge pressure, psia
Ahe= plate finish space, sq. in.
Ace = cylinder crank finish space, sq. in. = Ahe- rod space
Inertia Force
It is the summation of the merchandise of all reciprocal lots (piston and rod assembly, and crosshead assembly together with pin) and their individual acceleration.
Combined Rod Load
It is the pure mathematics total of Gas load and Inertia force act on the crosshead pin.
Fcombinedrod load=Fgas load + Finertia
As per API 618. The combined rod hundreds and therefore the gas hundreds shall be calculated for every 5-degree interval of 1 rotating shaft revolution.
The purpose at with the combined rod load modification it magnitude from Tension to Compression (negative to positive sign) or vice-versa is named as “Load Reversal”. This Load reversal act on the crosshead pin throughout every revolution.
During the look stage of the reciprocal mechanical device, the styleer can check that the length and magnitude of this load reversal shall be in step with the oil distribution design of the crosshead bushing so as to take care of correct lubrication. This to avoid crosshead pin failure.
Reciprocating mechanical device connecting rod Load Calculation methodology
For calculation of piston, rod load is incorporates 3 steps.
Step 1
First, we discover the gas load act on the piston. As mentioned higher than by exploitation the higher than 2 equation the gas load act on the piston is calculated for each 5oof one crank revolution. The Blue color bar represents the gas load act on the piston. Refer graph below.
Reciprocating mechanical device connecting rod Load – Principle and Calculation methodology
Step 2
The inertia force act on the piston thanks to the all reciprocal lots. Refer graph below, the gas load is additionally calculated for each 5oof one crank revolution.
Step 3
It is the pure mathematics total of Gas load and Inertia force act on the crosshead pin. Refer graph higher than, the boundary line indicates the combined rod load for each 5oof one rotating shaft 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 In any case, the combined rod load shall not be above the most allowable rod load. In our example the most allowable rod load is 300kN. As we have a tendency to seen from the figure the combined rod load doesn’t reach the most allowable rod load throughout one complete revolution.
In some case, the mechanical device shall be capable of handling short length excursions of operation involving a load increase up to 100 percent higher than most|the utmost|the most} allowable continuous combined rod load and/or maximum allowable continuous gas load.
Conclusion
Both the mechanical device manufacturer and therefore the client ought to make sure the Rod load most price to understand the reciprocal mechanical device limits. Also, the rod load curves and peak rod load/reversal values offer a crucial insight to predict the health of a reciprocal mechanical device.
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