Welcome back to Compressor University!
Here is the second installment from the Atkins, Hinchliff and McCain article. This month they continue their review of reciprocating compressor load limit definitions.
Robert X. Perez
History of "Rod Loads"
The 1st Edition of API-618 was published in 1964 (34 pages). It included no definition of what the term "Rod Load" meant. However, the data sheets did call for the compressor manufacturer to specify the "Max Allowable Rod Loading" and "Rated Rod Loading." So the definition of what that meant was left up to the compressor OEM.
In the 1963 edition of the Ingersoll-Rand (IR) frame ratings guide, the piston loads were defined. This document stated that piston load is frequently referred to as "rod load," which is a misnomer as it implies that the piston rod is the only limit in the establishment of a compressor load rating. It defined the piston load as the nominal pressure at the cylinder flange times the area of the piston.
These loads were easily calculated from simple equations (available in Part One). It went on to say that the actual rod load would include the effect of inertia and valve losses, but these effects were considered in the piston rod load ratings, i.e. the piston rod load ratings were necessarily conservative. This approach served the industry well, but perhaps resulted in "over-designed" machinery.
This was before the advent of electronic calculators and digital computers, so combined rod load was tedious to calculate. The practice at the time was to look at and report simply the nominal gas load only with no valve losses or inertia loads considered. On rare occasions if it was judged necessary due to a combination of high gas loads, high inertia forces and high volumetric efficiency (which can cause the gas load and inertia load to be additive), a manual calculation of combined rod load (gas + inertia + valve losses) would be done.
This would consist of drawing a PV card including valve losses, using a planimeter and slide rule to determine area (horsepower) and gas pressures at discrete degrees of rotation increments. Then inertia forces were calculated at each point and added to determine the combined rod load at the crosshead pin, forces in the connecting rod and crankshaft, and torque on the crankshaft. For a six-throw compressor it would typically require six engineers (one cylinder each) and one week to perform this task.
The 2nd Edition of API-618 was published in 1974 (39 pages). The committee pushed the compressor manufacturers to advise how rod loads were calculated and ensure that everyone would calculate rod loads the same way. This established the term "allowable rod load" and "actual rod loading." The actual rod load was defined as the force due to the differential pressure across the piston plus the inertia of the reciprocating parts transmitted through the piston rod. It also stated that the actual rod load calculated on the basis of cylinder relieving pressure (RV setting) shall not exceed the vendor's maximum allowable rod load.
By this time mainframe computers and programmable calculators were widely used. This allowed for more precise engineering calculations and the elimination of some of the conservatism in the design process. Practice was to calculate compressor performance and "gas load" using a programmable calculator, since the computations were relatively simple. Basic compressor sizing and feasibility studies used these methods.
The final performance, including actual rod load (combined rod load), was obtained using mainframe computers (punch cards, overnight batch processing, etc.). Gas loads were still calculated and reported based on nominal cylinder flange gas pressures, but actual rod load included the effect of valve pressure drop and inertia loads. There was variability between various users and OEMs on the reference points used for the calculation of combined rod load. At IR and Worthington, the reference point was the crosshead pin, so all inertia outboard of the pin bearing was included in the combined rod load calculation. There was also inconsistency over the relief valve pressure. Some users and OEMs (including IR) used the final relief valve pressure rather than each stage RV setting.
In the 3rd Edition (1986), API-618 grew to 111 pages. The term Maximum Allowable Combined Rod Load (MACRL) was defined. The combined rod load was defined as the algebraic sum of the differential gas pressure on the differential piston area plus the inertia force. The reference point for inertia loads was defined as being at the crosshead pin. Additionally API established a minimum rod load reversal criteria (to ensure proper lubrication, 3 percent and 15 degrees), but that issue is outside the scope of these articles. Gas load still was reported based on nominal cylinder flange pressures and the relief valve pressure calculation was still inconsistent.
In the 4th Edition (1995) API-618 was at 166 pages. The calculation of rod load was defined much more precisely. The terms Max Allowable Continuous Combined Rod Load (MACCRL) and Max Allowable Continuous Gas Load (MACGL) were established. This was the first time that load limits based on running gear and load limits based on the stationary components were explicitly separated in the specification. Combined rod load was defined the same way as the 3rd Edition but with the clarification that it was to be at the crosshead pin and only the component in the direction of piston motion was included. Note that the load in the connecting rod is higher due to geometry.
Gas load was defined as being the gas pressure inside the cylinder (cylinder flange pressure less valve and passageway losses). Combined rod load and gas load had to be calculated every 10 degrees of rotation. These loads had to be calculated and must be less than the manufacturer's MACCRL/MACGL limit at the RV setting of each stage and the minimum pressure for each stage.
Computer capabilities had increased to the point that the combined rod load calculations were readily available using PC based software and most machinery analyzers had the capability of computing the combined rod loads in real time as long as the measured in-cylinder pressures and the weights of the various components were properly used and interpreted.
Glossary of Terms
Rated Rod Load (RRL): Term used in 1st edition of API-618 but without definition. At IR interpretation was gas only load not including valve losses. Was not to be exceeded on any normal operating load step (specified operating pressures, not relief valve setting).
Maximum Allowable Rod Load (MARL): Term used in 1st edition of API-618 but without definition. At IR interpretation was gas only load not including valve losses. Was not to be exceeded at any upset including final discharge relief valve pressure. Operation at rod loads exceeding the MARL voided the warranty.
Actual Rod Load: Term used in 2nd edition of API-618 with definition. Included gas + valve losses + inertia loads in the calculation, but did not define the reference point for the calculation.
Maximum Allowable Combined Rod Load (MACRL): Term used in the 3rd edition of API-618 with definition. Same calculation as actual rod load (included gas + valve losses + inertia loads). Load was defined at the crosshead pin. This is the max load that can be applied at any load step including final RV pressure.
Maximum Allowable Continuous Combined Rod Load (MACCRL): Term used in 4th and 5th edition of API-618. Similar definition as MACRL except load to be calculated every 10 degrees (5 degrees in 5th edition) and load at the pin is the component in the direction of the piston motion. This is the current definition of the rated load that applies to the running gear.
Maximum Allowable Continuous Gas Load (MACGL): Term used in 4th and 5th edition of API-618. Includes internal gas pressures inside the cylinder (flange gas + valve passageway losses). This is the current definition of the rated load that applies to the stationary components.
Internal Gas Load: Term commonly used with no API definition. Typically means the gas load based on pressure inside the cylinder, i.e. includes the valve and passageway losses. It is the same as the current API-618 definition of MACGL.
Crosshead Pin Load: Term commonly used with no API definition. Typically means the same thing as the current API-618 definition of MACCRL. It is referred to as the crosshead pin load to avoid the frequent misconception that the combined rod load is at the crosshead end of the piston rod rather than the API defined crosshead pin reference point for inertia calculations.
Total Rod Load: Term used by Ariel with no API definition. It is used to define the maximum allowable load in tension plus compression that can be applied. Note the Total Rod Load limit is typically less than the sum of the internal gas load limit in compression and in tension. Ariel also lists a Rod Load limit, which is defined as an internal gas load. A Combined Rod Load limit is not published; however, Combined Rod Load is calculated on the performance sheet and is used in determining acceptable rod load reversals at the crosshead pin.
Be sure to check back next month for the final installment of this series.
Presented at the 2005 Gas Machinery Conference in Covington, KY, October 2-5, 2005
Robert Perez is the author of the Operator's Guide to Centrifugal Pumps and website editor for PumpCalcs.com. He has more than 25 years of rotating equipment experience in the petrochemical industry and holds a BSME degree from Texas A&M University in College Station, a MSME degree from the University of Texas at Austin and a Texas P.E. license.
