Reversal load, API 618 requirements and how to control it in reciprocating compressor

1.??????What is reversal load?

Rod reversal is one of the most important parameters affecting a reciprocating compressor. The ideal reversal angle is 180°. If the reversal angle is zero or is insufficient, the pin and bushing assemblies will lack effective lubrication and cooling, such that they may overheat and possibly seize together.

Load reversal could be considered as a controversial topic for reciprocating compressors, related definition to this topic has been changed since first edition of API 618 published in 1964 (34 pages) up to 5th edition in which has been defined as “rod reversal: A change in direction of force in the piston rod loading (tension to compression or vice-versa), which results in a load reversal at the crosshead pin during each revolution.”

2.??????How to calculate Reversal load?

The load which is applied to the crosshead pin bushing is developed from two sources: the forces of inertia of the reciprocating piston, rod, and crosshead assembly, and the forces resulting from compression of the gas in the cylinder.

?The inertial load is that force which develops as a result of the weight (mass) of the piston, rod, and crosshead assembly (including piston rings, nuts, crosshead pin, and balance weights) being in reciprocating motion.

The gas load on each end of the piston is determined by finding the pressure inside each end of the cylinder at various points in the stroke. This pressure is then multiplied by the respective piston areas to give the head end and crank end gas loads. The total gas load is then found by adding the H. E. gas load and C. E.

The net load applied to the crosshead pin bushing is found by the algebraic summation of the inertial load and the total gas load.

?As shown in Figure 1, when the piston rod is in compression, the wrist pin will be on one side of the cross head bushings and most of the clearance will be on the opposite side. At this point, most of the oil runs into the larger clearance while the oil film thickness at the loaded side is very small, such that the pin and bushing are more effectively lubricated and cooled on the no-loading side.

At the same time, the wrist pin is on one side of the connecting rod bushing and, again, most of the oil runs into the larger clearance area. When the rod is in tension, the oil better lubricates and cools the opposite side of the pin and bushing assemblies, as shown in Figure 2.

The reversal angle corresponds to that angle through which the crankshaft rotates as the piston force reverses. A sufficiently large reversal angle is necessary to ensure lubrication and cooling of the pin and bushing assemblies. As the crankshaft undergoes one revolution, all of these loads vary from minimum to maximum values.

?3.??????How to control reversal load?

As the capacity is varied, the inertial and friction forces will remain unchanged, but the distribution of the gas force will change with the capacity regulation ratio. Therefore, it is necessary to introduce the regulation ratio into the gas force calculation in order to control the reversal load, we need to adjust the rod load. The net rod load can be controlled by altering the constituents which comprise the rod load - namely the inertial load and the gas load, We certainly must evaluate how changes in each affect the rod load.

a)???????Changing in Inertial Load

1)??Increasing the rotating speed of the compressor will increase the inertial load and the amount of the reversal. (See Figure 3)

2)? Adding weight to the crosshead and piston assembly will also increase the inertial load and. the size of the rod load reversal.

b)??????Changing in Gas Load

The effect of the gas load on the rod load is quite different from the inertial effect. Increasing the gas load may either increase or decrease the rod load reversal. In a single acting application, for instance, increasing the gas load on the operating end will decrease the reversal. On the other hand, using CE cylinder instead of HE cylinder in partial load (in 25% or 75% of flow for 5 step compressors or 50% of 3 step compressor with odd number of cylinder per stage) of double acting compressor would increase the reversal. Ample precautions must, therefore, be employed in making changes to the gas load.

???????I.???????????Cylinder Configuration Changes

a.???????The end selected for operation (in single acting applications) will affect the rod load reversal, Operating the crank end, which has a smaller piston area, will produce a smaller gas load and increase the reversal.

b.??????Operating the cylinder in a double acting configuration will obviously increase the gas load and the reversal.

c.???????The size of the cylinder bore will affect the reversal in either way. Smaller bores in double acting cylinders tend to decrease the reversal, but decreasing the bore in single acting configurations tends to increase the reversal.

d.??????Decreased cylinder clearance will increase the volumetric efficiency and increase the gas load. In single acting situations, increased gas load will reduce the reversal; and in small diameter double acting cylinders, the increased gas load will expand the reversal.

?????II.???????????Changing in Operating Condition

Increasing the no. of stages would lead to reducing the compression ratio which decreases the gas load and usually improves the rod load reversal. This can be accomplished by either lowering discharge pressure or increasing the suction pressure.

?4.??????Requirement of API 618 for reversal load

We are now fully aware of the necessity of avoiding non-reversing rod loads. And we have illustrated various ways of manipulating the loads to accomplish the needed reversal. Now we can totally understand why documents on section 6.6 of API 618 5th edition are needed and what we should be looking for while reviewing such documents. According to API 618, Mass Force Calculation and Rod Load Curve shall be submitted according to following conditions:

a.?At PSV discharge pressure of each stage & lowest specific suction pressure corresponding to each load step

b. Each step load (for 5 step compressors shall be submitted for 0-25-50-75 and 100% of flow)

c.?The combined rod loads and the gas loads shall be calculated for each 5-degree interval of one crankshaft revolution

d.?The internal pressure during the suction stroke is the suction pressure at cylinder flange minus the valve and gas passage losses. The internal pressure during the discharge stroke is the discharge pressure at cylinder flange plus the valve and gas passage losses

Bullet items:

-?If specified, the vendor shall furnish the data required for independent rod load, gas load, and reversal calculations.

-?If specified, the effect of valve failure on rod loads and reversal shall be calculated and furnished. The required specifics of this study shall be mutually agreed upon by the purchaser and vendor.

Requirement of API 618 for Rod Load Curve:

a.???????The combined rod load shall not exceed the manufacturer’s maximum allowable continuous combined rod loading for the compressor running gear at any specified operating load step.

b.??????The gas loading shall not exceed the manufacturer’s maximum allowable continuous gas loading for the compressor static frame components (cylinders, heads, distance pieces, crosshead guides, crankcase, and bolting) at any specified operating load step.

c.???????Some bushing designs (such as grooved bushings) have proven reliability with as little as 15 degrees of rod reversal at 3% magnitude. Simple bushing designs (un-grooved) may require a minimum of 45 degrees of rod reversal and a 20% magnitude. The manufacturer should provide the actual requirements to the purchaser at the proposal stage.

?5.??????Reversal load ameliorating guidelines

Realizing the principles discussed above, we should be alerted for a non-reversing rod load when an application contains one of the following conditions:

a.?Slow Speed Operation:

slow speed operation by itself is not necessarily a problem. But with other conditions present, slow speed could be a significant contributor to a low reversal rod load.

b.?Single Acting Operation:

Low reversing rod loads occur in single acting operation more than in any other situation. And single acting head end operation (SAHE) is always more susceptible to low reversals than single acting crank end operation (SACE).

c.?Small bore sizes in double acting cylinders:

approach a single acting condition and are non-reversal prone.

d.?Low volumetric efficiencies (VE) often produce:

non-reversals. Low VE's result from high clearances, particularly in unloading sequences where clearance is deliberately added. When performing unloading, one should always remember that SAHE is more susceptible to non-reversals than SACE. The head end pockets should be opened first to avoid the low reversal.

e.?High compression ratios are apt to produce low reversals.

f.?High cylinder pressures are natural for low reversing rod loads.

They usually mean high gas loads, small cylinder bores, and sometimes single acting operation- all of which are susceptible to non-reverse Is.

g.?Tail rod in double acting cylinder:

Using tail rode is one of the solutions which could be considered for a double acting compressor while the cylinder is low compared to rod diameter. However, according to API 618, this configuration only by written approval of the purchaser is applicable.

?6.??????Challenge

?Refer to the following diagram, which illustrates loads regarding a 2-stage, 2 cylinders per each stage, double acting reciprocating compressor with 5 step control. This diagram demonstrates rod load for one of the cylinders at 25% capacity. Does it conform to API 618 requirements? Which characteristic needs to be revised? and what is your suggestion for the lowest cost?