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A Note from Robert X. Perez:
Welcome back to Compressor University!
Here is the third and final installment from the Atkins, Hinchliff and McCain article. This month they complete their review of reciprocating compressor load limit definitions.
Robert X. Perez
User's Perspective
Various OEMs used other terms such as "Maximum Allowable Frame Load," "Maximum Allowable Gas Load," etc. but only MACCRL and MACGL are defined in API-618 and only since the 4th Edition (1995). It is not clear that all OEMs, users, analysts, operators, analyzer vendors, etc. recognize these terms and agree that reciprocating machinery should be rated in this manner.
Oxy Permian owns, operates and maintains in excess of 400,000 horsepower of compression in West Texas and Eastern New Mexico. This includes screw, centrifugal and reciprocating machines. Most of these machines are reciprocating compressors and range in age from a few months to several decades with the majority installed prior to 1986 (i.e. pre-3rd Edition of API-618). The OEM published "rod load" limits range from 18,000 to 225,000 lbf. These machines vary by service, manufacturer, speed, loading, installation and operating philosophies and yield an array of equipment configurations. Longevity of service requires many machines to be subjected to a variety of process conditions that result in full utilization of "rod load" capabilities applied to these machines at commissioning. Several factors affect the actual rod load on a machine including declining field pressures, process changes, improper operation of unloaders, valve degradation, ring failure, process upsets and machinery modifications.
Oxy Permian performs compressor analyses on the majority of reciprocating compressors at approximately six week intervals. This snapshot of compressor operation presents the facility with the machinery health at the time of data collection. The rod load is presented, generally in a "percentage of allowable rod load" format, and the facility makes maintenance and operation decisions based on various components of these health reports. The question is: What exactly are we looking at and how do we compare our measured "rod load" to the OEM recommended maximum? For practical purposes, a facility is able to measure the gas load on a compressor by using either peak pressures generated inside the cylinder (periodic measurements by experience analyst with portable equipment), or flange pressures (typical pressure gages, transmitters, etc.), along with the cross sectional area of the piston. If the flange pressures are used, then the resulting loads must be compared to allowable "flange loads", but that is not an API definition and all OEMs do not provide such allowable loads. If the appropriate reciprocating weights are known (piston, piston rod, nuts, rings, riders, crosshead, bushings, pins, etc.), then the inertial loads can also be calculated. These inertial loads can be added to the gas load to develop the combined rod load. However, the reciprocating weights are not always known as modifications may have been made over the years and the degree of recordkeeping may be in question. A thorough understanding of what the rod load rating implies is generally not apparent to the typical machinery analyst/engineer/operator. The analyzer software will typically display both gas loads and combined rod loads, but only one allowable value is available (see Figure 10).
Many times the combined rod load is lower than the gas rod load, but this is not always the case as will be shown later. The standard rod load reports (see Figure 11) do not compare the measured values to both MACCRL and MACGL.
Facilities must continually do more with less and are generally capital constrained such that we must obtain every pound of load capability, in addition to all available horsepower, from a given machine. As we pull liners from cylinders, install new cylinders and push the machines to the max, we have found that in some cases we do not actually understand the rod load limits of the compressor. Economic viability of a new project, whether revamping an existing machine, or adding either parallel or series compression, is usually based on three items: power availability, additional throughput and machinery limitations. It is imperative that the OEM be included when evaluating design options because as an end user, we are not always privy to all design limitations on a machine. For example, the original installation may have been designed with custom distance pieces on one or more throws, and that may limit the load carrying capability of the frame. The following examples serve to illustrate the issues at hand.
Performance Study to Evaluate Compressor Re-Rate
A group of compressors was being considered for a re-rate project to increase capacity. The economics of the project depended on (among other things) the capital cost to modify the existing compressors if the increased capacity resulted in overload conditions. The compressors in question were from two different OEMs and were pre-1995 (prior to API-618 4th Edition) vintage. The load ratings for one type of compressor were provided in terms of "Rated Rod Load" and "Maximum Allowable Rod Load.". The load rating for the other compressor model were provided in terms of "Rated Rod Load," "Rated Flange-to-Flange Load," "Maximum Allowable Rod Load" and "Maximum Allowable Flange-to-Flange Gas Load."
For brand "X," the Rated Rod Load was 150,000 lbs, and the Maximum Allowable Rod Load was 180,000 lbs. The performance study concluded that overload conditions would occur, based on comparing the calculated pin loads to the 150,000 lb limit. It was later clarified that the MACCRL was 180,000 lbs and the MACGL was 180,000 lbs for this application.
The brand "Y" machines had a Rated Rod Load of 175,000 lbs, a Rated Flange-to-Flange Gas Load of 187,500 lbs, a Maximum Allowable Rod Load of 210,000 lbs and a Maximum Allowable Flange-to-Flange Gas Load of 225,000 lbs. The performance study also concluded that overload conditions would occur based on comparing the pin loads to the 175,000 lbs limit. It was later clarified that the MACCRL was 210,000 lbs and the MACGL was 225,000 lbs for this application.
The predicted overload situation initially led to proposed modifications to the compressors. This would have added a significant capital cost to the project. After the rod load ratings and operating conditions were reviewed with the OEM (and the current API definitions were applied), it was verified that there was not an overload condition. The project was delayed while the overload issue was resolved and later deferred, due to market conditions.
Combined Load Exceeds Gas Load
Many users mistakenly assume that the combined rod loads (gas plus inertia) will always be lower than the gas loads. This is not true for low ratio (high volumetric efficiency) applications. As an approximate rule, if the discharge volumetric efficiency (VE) exceeds 50 percent, the gas load will reach a maximum prior to 90-deg, while both inertia load and gas load are same sign thus are additive. If the discharge VE is less than 50 percent then the gas load does not reach a maximum until after 90-deg and so it is opposite in sign to the inertia load and the combined rod load will be less than the gas load. This is illustrated in Figure 12, which shows measured rod load data for a 250-rpm single-stage compressor in natural gas transmission service.
Distorted Pressure Measurements = Distorted Rod Loads
Another common mistake is to report distorted rod loads based on distorted pressure measurements. This is most often due to the "channel resonance" effect present in nearly all in-cylinder pressure measurements. This effect is illustrated in Figures 13 and 14. The rod load plot without channel resonance correction is shown in Figure 13, while the corrected plot is shown in Figure 14. The reported rod load is higher when the channel resonance is present.
Conclusions
Compressor rod load ratings are often misunderstood and misapplied. It is important to understand that the API definitions of MACCRL and MACGL are not actually rod loads, but refer to crosshead pin loads and gas loads, respectively. The API definitions help to avoid confusion, but these ratings are not always available for pre-1995 vintage machines.
"Measured" rod loads are actually computed rod loads based on measured pressures. The forces based on the measured pressures are combined with inertia forces based on the weights of reciprocating components inputted into the analysis software. If the pressure measurements are distorted and/or the reciprocating weights are not accurately known, then the combined rod loads reported will be erroneous.
There is some logic in using the simplified gas rod load calculations presented in equations 1 and 2. The trends will be correct (i.e., higher differential pressure results in higher rod load). However, if nominal flange pressures are used to rate a compressor, care must be take to include enough margin to account for the maximum possible in-cylinder pressures due to pressure drop, valve losses, pulsation and valve dynamics. These effects vary for each application.
The user must make a decision when a compressor is revamped on whether to use current API definitions and ratings, or the ratings in effect when the machine was first installed. Again, it is imperative that the OEM be included when evaluating design options because as an end user, we are not privy to all design limitations on a machine.
Presented at the 2005 Gas Machinery Conference in Covington, KY, October 2-5, 2005
Tags: Compressor University , Compressors
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