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Section 2: Evaluating The Methods


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For compressor stations with UCPs based on the Fast and Easy method, self-preservation suggests only visiting those stations when their units are shut down. This method is often implemented by system integrators (SI), whose programmers curvefit a handful of calculated points, and then apply a few generic rules (such as maximum pressure differentials and maximum compression ratios) to keep the units running and “safe”. This simple method typically does not pre-estimate interstage pressures before load steps are changed or look ahead to determine if changing unit speed creates pin reversal issues. For single-stage units that operate over narrow ranges, use only double-acting cylinders and do not apply too much clearance to either head ends or crank ends. This approach is not completely without some merit. It is commonly implemented in UCPs for many slow-speed, transmission and gas gathering applications. The real problems begin when this Fast and Easy method is applied to more generic units and/or wider operating ranges with the belief that this simple method is actually good to use for all applications. It’s not.

The Covers 90% of Concerns approach involves more calculations regarding unit performance and safety, and is the most frequently used method by experienced end-users and their elected and experienced system integrators. The calculations and methods for determining safe operating limits are more robust, better understood, and are based on actual modeling equations that calculate performance in real-time versus a static curvefit approach. Here, thermodynamics are considered, interstage pressures are reasonably predicted, rod loads

“The industry does offer a couple approaches.”

are better predicted, and the ability to model both double-acting and single-acting compressor modes is standard. This approach works well for gas gathering, gas boosting, transmission, injection, withdrawal, and even some process services. However, this method typically only works well for slow-speed units. For those units, the legacy compressor manufacturer typically reviewed the operating ranges (and often indicated the reviewed, and thus allowed, safety ranges on the unit nameplating) for complex issues like internal rod loads, pin loading forces, degrees of pin reversal, and crank pin forces. The control panel could effectively ignore those issues and concentrate on overload, high pressures, static (gas force) rod loads, low volumetric efficiencies, and high discharge temperatures. The problem with this method begins when it is applied to high-speed units under the belief that a method appropriate for slow-speed units is generically valid for high-speed units. It’s not.

The need for a third option, an OEM-based method is evident. It is a method that covers unit performance and safety according to all OEM limits and methods. While this may seem like the obvious choice for all units to use, keep in mind that a higher level of modeling complexity brings about a higher dollar cost. Additionally, how does one model unit performance and safety for each OEM’s line of compressors if that knowledge is not readily available? How can one expect a low-level (and computationally slower) PLC to handle the complex mathematical methods that take even a high-end desktop PC a few seconds to compute? Fortunately, the industry does offer a couple approaches to satisfy this option. The first approach is via unit-specific PLC algorithms based on OEM performance and safety limits programmed directly into the UCP. The second approach is via use of an add-on micro-controller that computes complex performance in real-time using the actual OEM-specified equations and methods.

Download the full PDF here or read the next article in the series:  Section 3: The Importance of Using the OEM Method

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