Consider modular reed valves for reciprocating compressors

The main function of reciprocating com­pressor valves is to control flow in and out of the compressor cylinders. Valves are critically important components for reciprocating compressors; their functional integ­rity usually governs the availability, reliability and efficiency of these positive gas-displacing machines. Virtually all manufacturers’ and users’ statistics place valves at the top of failing components in reciprocat­ing compressors.

Four or five types of valve configurations are nor­mally used in reciprocating compressors. Except for suction valve unloaders, which use mechanisms to physically keep a valve open during part or the entire stroke length (displacement) of a piston, valves are actuated by differential gas pressure. Valves open when the pressure on one side of the cylinder is greater than the pressure on the opposing side.

The compressor designer’s and knowledgeable user’s choice of valve depends largely on the characteristics or parameters of a process application. While valve type and size are being specified by the compressor manufacturer, the user-purchaser must be involved in valve selection. Selection involves trade-offs. Perhaps a given configurations has better efficiency but needs more maintenance or the manufacturer believes that unless the marketplace demands it, there is no incentive to change the present spare parts-oriented business model. Accordingly, he continues to provide valves that do not represent best available technology.

Modular reed valves

Although new valve designs are not entering the market­place every year, some new entries do exist and are well worth considering. In fact, because owner-purchasers are likely the most reliability-focused of the various enti­ties dealing with valves, users will benefit from a fuller understanding why modular reed valves have found rapid acceptance since 2014; many are now installed in oil refineries and hydrogen compressors with power ratings approaching 20,000 hp (see Figure 1).

Figure 1: Modular reed valve Courtesy: Zahroof Valves

In traditional (legacy) valves, the gas must travel around corners, which causes efficiency decays due to pressure loss. However, in a modular reed valve, the gas path is unobstructed. Also, the differential pres­sure needed for a valve to open is considerably less in modular reed valves than in legacy valves. When these parameters combine with demonstrable life extensions, the overall advantages are difficult to ignore.

Since many factors will affect reliability, user-to-user comparisons can be difficult. We advocate that reliability-focused owner-purchasers look for dem­onstrated experience in similar services elsewhere. As an important aside, a cooperative vendor or valve manufacturer will not withhold their reference lists from reliability professionals engaged in due diligence.

Design considerations

Considering modular reed valve design, the underlying design principle of a modular reed valve is captured in the term “straight-through flow” or StraightFlo. Although these valves are placed in the category “reed valves,” their use of high-performance polymers and truly modular elements of construction distinguish them from most other valves. at high velocity past two or more corners (see Figure 2).

Fig. 2: Traditional valves and depiction of flow. Courtesy: Zahroof Valves

The StraightFlo valve has much lower pressure drop; it consistently results in power savings of 0.5% or higher (see Figure 3). The manufacturer submitted data from a highly instrumented case study indicat­ing a 41% reduction of power losses in the valves of a 1,000 hp integral gas engine compressor in natural gas service. Specific power (bhp/volume flow) was improved by 9.3% and similar results were achieved at other compressor stations.

Fig. 3: Two modular reed elements illustrating straight-through flow path. Courtesy: Zahroof Valves

In the pursuit of comparison, a well-managed user-purchaser usually applies the principles of machinery quality assessment (MQA)(ref. 2). While carrying out their MQA tasks, reliability professionals verify that the vendor’s material selection meets the purchaser’s gas conditions, operating temperature criteria and anticipated range of power losses. Obtaining a war­ranty-backed gain of 0.5% in overall power efficiency in a 6,000-kW reciprocating compressor at $0.06/kwh and 8,760 operating hours per year is worth more than one might think. Considering the value of downtime avoidance will further enhance the often very attrac­tive payback.

A competent valve manufacturer will explain all about design compromises, gas velocity limitations, the meaning of valve lift (distance from seated to fully open, absence of valve flutter), tolerance for liquids and solid impurities carried in the gas and overall valve operation in clean versus fouling ser­vices. Recall that there are bone-dry gases in some, and oil carryover in other services. Explore if and how a compressor valve tolerates these extremes. Find out before a sequence of premature failures tells you that experimentation is rarely acceptable when important assets are involved.

Examining modular reed valves

The inventor chose the generic term “modular reed valve” for compressor valves designed and constructed as depicted in several of our illustrations. Adaptations are available of different cages, stacked valve models, unloading mechanisms and other configurational choices. However, the high-performance plastic, reed-containing components depicted in Figure 1 are com­mon to all; even their external dimensions are identical. Where traditional valves experienced severe fouling, a StraightFlo drop-in replacement always showed con­siderably less fouling in the same time frame. Because of its largely self-cleaning action, the drop-in replace­ment greatly extends the run length capability of other valves. The novel valve concept allows field-cleaning by a relatively inexperienced labor force and usually takes less than 15 minutes.

We ascertained ease of service, improved compres­sor flowrates, servicing possible by lower skilled work­force members, above-average operating performance over a very wide speed range and generally well above-average availability/reliability. While the first applica­tions were in upstream facilities (oil exploration and gathering), StraightFlo valves have made significant inroads in downstream facilities such as oil refineries and petrochemical plants. The uniform construction of the valve internals facilitates spare parts procurement and management to an unusual degree.

The flow area ratios of two widely used valves were compared with three modular StraightFlo valves; the percentage increases in flow areas were judged to be quite significant. Elementary physics explains that, for a given volume of gas, larger flow areas have lower pressure drop than smaller flow areas. Lower maximum pressures in cylinders reduce pressure ratios by a small amount. Together with inherently larger areas of gas passage, these ratio reductions save energy and increase compressor throughput.

Final thoughts

We were able to verify that the success of these valves is based on well-instrumented factory tests as well as field tests. We ascertained the various reports did not refer to comparisons of brand-new modular reed Straight­Flo valves with old and/or worn valves. Instead, the comparisons were for unused old-style valves versus unused new-style valves.

The tests also showed sizeable improvements in spe­cific energy efficiency (bhp divided by volume flow), no valve flutter, low noise and vibration, extremely low-pressure differential needed to activate valves, greatly extended trouble-free operating time and realization of significant maintenance savings.

References

Bloch, H. P., and H.G. Elliott; “New Approaches to Compressor Technology,” (2021) De Gruyter Publish­ing GmbH, Berlin, Germany, ISBN 978-3-11-067873-4

Bloch, H. P., and F. K. Geitner; “Compressors: How to Achieve High Reliability & Availability,“(2012), McGraw-Hill Publishing Company, New York, NY, ISBN 978-0-07-177287-7

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