Recip, screw or centrifugal compressor? An end-user’s perspective

Picture courtesy Siemens (Dresser-Rand)[/caption]

The end-user has to balance technical and non-technical requirements in order to finalize on a particular selection. Non-technical requirements such as familiarity with the machine, delivery, costs, maintenance, parts, quality, and operability can make or break the project. The ability of the compressor selection to handle off design operation (including startup and shutdown) has to be considered. How many times has the reader crossed their fingers and said “If we can just get it started, it will run great?”

Oversight of these nuances can have consequences, some not becoming apparent until the equipment is installed. It should be recognized that while technical requirements are critical for long term reliable service, the engineer may find themselves having to select a machine that is best for the project and not so much the application. The project always has technical constraints, but also commercial and schedule considerations to take into account. Also important are items such as availability of replacement parts, craft skills available for maintenance, repair facility options, staffing, and automation.

This article contains excerpts from the paper, “Your Gas Compression Application – Reciprocating, Centrifugal, or Screw?” presented at the 2016 Turbomachinery Symposium by Greg Phillippi of Ariel Corporation, Tim Manthey of Aerzen USA, Jonathan Sutter of Elliott Group, Ben Williams of Ariel Corporation, Bruce McCain, an engineering consultant.

Ultimately, the end user has to make a technical compromise to select the best machine the project can bear that will be delivered and installed in the promised time, and without exceeding the budget. Everything about machinery selection can be lumped into one of these three categories: Quality, Delivery, and Price. Whether a technical or “soft” issue, quality, delivery, and price should always be brought into the discussion.

As simple as it may sound, this entire process begins with having a thorough understanding of the project requirements. Input from operations personnel provides insight to operational deviations. For example, 95% of the time a gas gathering service in the upstream sector may carry an acceptable amount of water (or other liquids) entrained in the gas, but the other 5% of the time (18 days of a year) the amount of liquids in the gas is beyond the capacity of the scrubbing system, as initially designed. This additional liquid loading could be due to low ambient temperature, high winds, blowing snow, well treating chemicals, pigging of the pipeline, etc. If a reciprocating compressor is applied here, the user may not be able to replace compressor valves as fast as they fail, or scrubber high level alarms won’t clear. Considering a centrifugal compressor with a water wash system, the end user may exhaust their reverse osmosis water supply, or the system freezes and there are extended periods without online washing. Alternatively a screw compressor is installed in a gathering service where a new well is tied in with a composition very different than the balance of the system. This can (and has) caused significant lube carryover problems and other lube issues that prevent the screw compressor from operating. In all cases, for one reason or another, the compressors won’t run reliably.

There may not be an easy solution for each of these problems every time, but the project staff should understand “off design” conditions and practical future expectations. The operations team must be involved when deciding how to handle off design issues that have occurred in the past. More times than not, in the upstream sector, off design becomes the new normal. It is better to profit from the mistakes of others, or at the very least try to polish up the crystal ball and understand what the machine will be asked to do once project personnel have moved on to other endeavors. This shouldn’t be construed as a complete review of considerations but rather a few points to keep in mind when deciding which type of compressor to select.

The upstream sector uses a large number of reciprocating compressors. One of the reasons is the high degree of flexibility, or operating range. Reciprocating compressors can be very forgiving for a wide range of operating conditions, so long as those conditions are taken into account during the early phases of the project. It is recommended to seek the advice of operating personnel as they have a good understanding of what happens in reality. Many installations are unmanned, either initially or are converted to unmanned. The degree of potential automation must be taken into account. If it’s reasonable to believe it will eventually be unmanned, then one should design it as such.

One of the biggest problems end users of reciprocating compressors deal with is the change of seasons. The first freeze of the year, or a late freeze in the spring, causes problems if the interstage coolers (finfans) have manual louvers configured for warmer ambient conditions and/or constant speed fans. One successful option is to have automated louvers with variable speed fans on temperature control.

This latter option is more costly to implement, but successful in avoiding condensation (water or hydrocarbon) that can carry over into the compressor causing downtime if it gets past the scrubber. Conversely, blowing sand can cause problems with automated louvers. Again, consider everything. A great package design for one location may be a poor design for another.

Suction temperature set points should have a large, practical margin to prevent problems due to pressure drop across bottles, orifice plates, nozzle, passages, and valves. There may not be a liquid level in a suction scrubber but this does not guarantee the avoidance of condensation very close to (or inside) the compressor cylinder due to the Joule-Thomson (JT) effect across the listed components. Properly sized recycle lines allow for startup in cold conditions. Many installations run just fine, if the operation can ever get started. Having a properly sized and placed recycle line with a temperature controller actuating the valve will allow the user to slowly heat the system with relatively hot discharge gas. Properly sized recycle lines allow for startup in cold conditions. Many installations run just fine if the equipment can get started. Having a properly sized and placed recycle line with a temperature controller actuating the valve will allow the user to slowly heat the system with relatively hot discharge gas.

Considering winter operation is not enough- one should consider winter startup from a fully depressurized, cold system. Compressor and engine frame heaters, oil circulation systems and the time it takes to heat those systems (including off skid piping) up should be reviewed.

Reciprocating compressors can handle a large range of conditions with the proper design. Reciprocating compressors generate gas pulsations at harmonics of running speed. These pulsations can couple mechanically with piping, fittings, vessels, nozzles, and if coincident with mechanical natural frequencies can amplify significantly causing component failures, generally fatigue related.

There are two schools of thought on this topic: vibration control, and pulsation control. Vibration control seeks to hold components in place with clamps and supports, and design components such that Mechanical Natural Frequencies (MNF’s) have adequate separation margin away from excitations (mechanical and acoustic). Pulsation control minimizes amplitude of pulsation and filters out frequencies above a specific value (Helmholtz Frequency). In reality, both are important. Predominate mechanical excitations on reciprocating compression generally occur at 1x and 2x running speed due the reciprocating masses (pistons, crossheads, etc.), while pulsations occur at all harmonics (1x, 2x, 3x, 4x, etc.).

Predicting MNF’s is not an exact science and has significant error, depending on stiffness of components/shapes, manufacturing, assembly, etc. When you take separation margins between excitations (mechanical and pulsation) into account, there is “nowhere to hide

Using a variable speed driver further complicates the operation by having an even broader range of potential excitation frequencies. Understanding this concept is critical to the reciprocating compressor package engineer and user. All practical operating scenarios must be taken into account when designing the pulsation control system. Pulsation is important and must be controlled to maintain long term reliability.