This Enalysis Tip details Detechtion Technologies’ recommended procedure for determining the shutdown, alarm and control settings for a reciprocating compressor. Compressor safety shutdown systems consist of pressure, temperature and vibration sensors combined with a set of annunciators and switches. Alarm and shutdown triggers are applied to critical sensors to prevent operating under any conditions that would exceed compressor rated limits. Alarms provide a warning to an operator that a limit is being approached, while triggering a shutdown will immediately cease the operation of the compressor. Therefore, it is essential that each shutdown is set such that the compressor is always protected, while still allowing a sufficient operating range so as not to limit gas production.
The protection of any compressor meeting the minimum requirement for safety shutdowns depends on the static shutdown trip settings on the following monitored process parameters:
The operating range can be increased by adding alarm and shutdown settings on the following monitored process parameters:
As part of Detechtion’s standard services, your Engineering Account Manager will be happy to develop a Startup Package to ensure that your compressor is properly protected. A new Startup Package should be developed every time the compressor configuration is modified or the final stage PSHH is changed.
A Detechtion Startup Package includes:
Standard compressor safety shutdown systems use static pressure and temperature trip points. This requires that a “worst-case” scenario of operating conditions be consider when determining safety shutdowns. The most important consideration in determining all shutdown and alarm settings is the required final stage high discharge-gas pressure shutdown, PSHH. This value must be high enough to accommodate anticipated increases in discharge pressure. However, a PSHH that is unnecessarily high will negatively impact the operating range by limiting other shutdown settings. A high PSHH will result in an elevated PSLL, as discharge temperature, volumetric efficiency and rod load limits will be reached sooner due to the increased compression ratio. This will necessitate a higher recycle valve control point, minimizing the ability of the compressor to pull down suction pressure and stimulate declining fields. A high PSHH will also reduce maximum throughput by lowering the high suction control point to prevent high driver power requirements.
All control, alarm and shutdown points calculated based on a static PSHH will be conservative whenever the operating discharge pressure is less than the PSHH.
Assumed Stage 1 and interstage temperatures also limit the upper and lower range of operation. Elevated temperatures will result in high discharge temperatures being calculated earlier on declining suction pressure and low temperatures will calculate high engine loads at lower suction pressures when high throughput is desired.
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