Compressors Reliability

Matthews, Terryl, Field Performance Testing to Improve Compressor Reliability, Proceedings of the 10th Turbomachinery Symposium, College Station, TX Texas A M University, 1981, pp. 165-167. 

A specification should outline to the bidding vendors just what it is you want to accomplish with the equipment and how reliable it must be. Compressor reliability can be achieved if it begins prior to vendor design and fabrication. Therefore, a truly reliable piece of equipment begins in the specification. 

Brown, Royce N., Can Specifications Improve Compressor Reliability Hydrocarbon Processing,, luly 1972, pp. 89-91. 

The preceding discussion covered quite a number of topics relative to compressor reliability. A brief summary of the items covered may help the reader recap some of the thoughts presented. Issues that commonly play a major role in the search for reliability of compressors include ... 

Load factor is particularly important with air-cooled machines where sustained full-load operation results in an early build-up of deposits on valves and other parts. This build-up increases the frequency of maintenance required to maintain compressor reliability. Intermittent operation is always recommended for these units. The frequency and duration of unloaded operation depends on the type, size, operating pressure of the compressor. Air-cooled compressors for higher than 200-psig-pressure application are usually rated by a rule that states that the compressing time shall not exceed 30 minutes or less than 10 minutes. Shutdown or unloaded time should be at least equal to compression time or 50 per cent. 

Large centrifugal compressors have proven to be highly reliable in evaporator systems. Most operational problems have been caused by improper matching of compressor and evaporator and excessive entrainment. Several actions are recommended to maximize compressor reliability ... 

In order to improve compressor reliability it is critical to choose the correct materials when operating in harsh environments. Using the proper materials, centrifugal compressors can be designed to handle corrosive and erosive gases. In order to choose the correct materials, all possible operating conditions must be accurately defined. 

Reliability is Enhanced by Adding a Second Brayton Loop only if Turbine and Compressor Reliability below 97%... 

Table 7-2 System Reliability Results for Turblne/Compressor Reliability of 97%.18...

Table 7-3 Reliability Results for Turbine/Compressor Reliability of 99%.19...

The Guidelines for Process Equipment Reliability Data with Data Tables covers a variety of components used in the chemical process industry, including electrical equipment, analyzers, instrumentation and controls, detectors, heat exchangers, piping systems, rotating equipment (pump, compressor, and fan), valves, and fire protection systems. 

The OREDA Offshore Reliability Data Handbooks covers a variety of components used in offshore oil drilling and platforms, including gas/fire detection systems, process alarm systems, firefighting systems, pressure relieving systems, general alarm and communication systems, evacuation systems, process systems (vessels, valves, pumps, heat exchangers, and compressors), electrical and utility systems, and drilling equipment. 

Numerous applications where the recovery of power is important are being explored and exploited to an increasing degree. These are classified as turboexpander applications because of the importance of reliability and high efficiency. Turboexpanders meet these requirements and are available in the needed capacity ranges. A 5,000 hp (3,727 kW) compressor-loaded turboexpander is shown in Figure 2-10. 

Caution should be taken to prevent excessive acceleration. One solution is to provide for opening of the compressor discharge vent valve to increase the compressor flow and increase blower horsepower. This is effective with a centrifugal compressor, however, not with an axial compressor unless the compressor is provided with adjustable stator vanes that are reliable. This can be seen from the performance curve. The head versus flow curve for a given vane setting is extremely steep and opening the vent valve is ineffective. However, if the vanes operate fully open and the vent valve opens, the combined effect is satisfactory. The vent valve must not be oversized. 

Digital control methodologies similar to the Elliott Digital System program evolved from successful and proven reliable high-speed air compressor management and control approaches. 

Standard turboexpander-compressor sets designed and manufactured by competent manufactureres embody many of the reliability improvement and maintenance cost reduction features inherent in API (American Petroleum Institute) standards for other turbomachines. To achieve close compliance, however, the owner/purchasers must know their specific needs or requirements. Nevertheless, it may not be practical, feasible, or economically justified to achieve full compliance with API specifications, which were generally devised without giving thought to the peculiarities of turboexpanders and their sometimes differing operating environments. 

Attached to the expanders, the compressors provide an additional 60 psig without energy cost outlay. Further compression ultimately raises gas pressure to 34 bar. MESA estimates that the arrangement at its Satanta plant saves 45 kW (60 hp) while reliably handling the daily production of natural gas. 

This standard presents and describes features that are desirable for the user to specify in order to select a gas turbine that will yield satisfactory performance, availability, and reliability. The standard is limited to a consideration of the basic gas turbine including the compressor, combustion system, and turbine. 

Webb, S.G., and Calistrat, M.M., Flexible Couplings, 2nd Symposium on Compressor Train Reliability, Manufacturing Chemists Association, April 1972. 

Compressors are usually high-cost items, but easily correlated by brake horsepower vs. S/horsepower. Variations in engine-driven reciprocating compressor prices can be caused by the type of driver, the speed (the slower the speed the more costly, but the more reliable), the total discharge pressure, and the size. 

The ejector can first be identified as having no moving parts (see Figure i-10). It is used primarily for that feature as it is not as efficient as most of the mechanical compressors. Simplicity and the lack of wearing parts contribute to the unit s inherent reliability and low-maintenance expense. 

The basis for evaluation of piston speed varies throughout industry. This indicates that the subject is spiced with as much emotion as technical basics. An attempt to sort out the fundamentals will be made. First, because there are so many configurations and forms of the reciprocating compressor, it would appear logical that there is no one piston speed limit that will apply across the board to all machines. The manufacturer is at odds with the user because he would like to keep the speed up to keep the size of the compressor down, while the user would like to keep the speed down for reliability purposes. As is true for so many other cases, the referee is the economics. An obvious reason to limit the speed is maintenance... 

Davis, H., Effects of Reciprocating Compressor Valve Design on Performance and Reliability, Presented at Mechanical Engineers, London, England, October 13, 1970 (Reprint, Worthington Corp., Buffalo, NY). 

Historically, the most popular driver for the centrifugal compressor has been the steam turbine. Steam turbines can readily be speed matched to the compressor. Prior to the upsurge in energy costs, reliability, simplicity, and operational convenience were the primary factors in driver selection. The steam turbine, with its ability to operate over a relatively wide speed range, was ideal for the centrifugal compressor, which could be matched to the process load by speed modulation. 

The inverters are either voltage source or current source (see Figure 7-7a and b). There are other variations, but they apply to drivers smaller than the ones used with compressors. However, pulse-width-modulated (PWM) (see Figure 7-7c), transistorized units are less complicated and are relatively maintenance-free with reliable units available to at least 500 hp. For all but the smaller compressors, the current source inverter is the one typically used. With a six-step voltage source, a rule of thumb has been to size the motor at two-thirds of its rating so as not to exceed the insulation temperature rise. For current source motors, the output torque is not constant with decreased speed, which fortunately is compatible with most compressors, as torque tends to follow speed. For current source drives, one needs to upsize the motor captive transformer by approximately 15% to account for harmonic heating effects. 

In some applications (usually high pressure compressors using oil film seals) alternative pump schemes should be considered. It may be that the desired seal pressure is not achievable by one set of pumps or the quantity required by the seal is small relative to the main pump capacity. There are (imes when booster pumps are needed however, if the reason is energy, it would be worth reviewing the economics very carefully, because reliability tends to suffer with the booster. The booster pumps are paired into a main and standby and are configured to take suction fro lower pressure system. Sufficient interlocks have to be supplied drivers so that if the main pumps shutdown, the boosters come ... 

Neal, D. F, Centrifugal Compressor Mechanical Testing, Pre.sented at The Third Compressor Train Reliability Symposium sponsored by 1 ngi necring Advisory Committee, Manufacturing Chemists Association. April, 1973. 

It is not unusual to find more than one compressor suited to the job. Additional factors, such as inherent reliability and efficiency of the v arious types, should be considered. First cost will be an unavoidable factor that must be addressed. If, after consideration of all pertinent factors, there is still an overlap, then it may be advisable to inquire about more than one type of compressor. Normally this doesn t happen, because once the types are reviewed, some factor sways the decision. If, however, more than one compressor will be inquired, a specification should be prepared for each. It is quite difficult to write a comprehensive multipurpose specification. While there are some areas that will be the same, a repeat of each document will make evaluation much easier and cleaner later on... 

One way to decrease your reliability and cause problems is to not men tion fouling potential in centrifugal compressor specifications if proce.s.s experience exists. The experience may exist but not be recognized... 

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