Surge Control Alternatives for Refrigeration Systems

Unlike the natural gas compressors which rely upon recycling the after-cooled compressed gas to compressor suction for surge prevention, the surge control in refrigeration systems is quite different because the refrigerant vapors are compressed only to their saturation pressure for dew point temperature, which when cooled for recycling immediately condense into a liquid. Consequently, the hot compressed refrigerant vapors, also known as the hot gas, before any cooling are recycled to compressor suction to make up for any loss of the process chilling load that reduces the amount of available refrigerant vapors to satisfy the minimum flow to the compressor suction, thus avoiding the phenomena known as “surge”.

Three-Stage Refrigeration Cycle

A three-stage propane refrigeration system [1, 2] is adapted into Figure 1 to illustrate a typical refrigeration cycle.

”]As shown in Figure 1, all three process chillers are located on the 1st stage and there is no process side chilling load at the two economizer levels. The shown schematic is a simplest illustration of a three-stage propane refrigeration cycle [1, 2].

The compressed refrigerant vapors are condensed with ambient air. The high pressure (D-0116) and low-pressure (D-0115) economizers separate the flashed vapors that enter the third and second stages of the refrigerant compressor, respectively. To protect the compressor from any liquid carry over, each stage has a suction drum, and all vapors entering the compressor must pass through these drums (D-0112, D-0113 and D-0114).

Surge Protection with Hot Gas and Quench Liquid

So how do you provide a cool vapor stream to prevent the refrigerant compressor from surging in order to keep the discharge temperature from getting too high? To address this challenge, hot compressed vapors from refrigerant compressor discharge are used to vaporize liquid refrigerant, in a similar manner to how the process stream is used to vaporize refrigerant in the chillers.

In the refrigeration cycle of Figure 1, the hot gas from compressor discharge flows into each suction drum as determined by the anti-surge controller to prevent each compression stage from surging. To cool the hot recycled vapors, a temperature controller introduces quench liquid directly from the refrigerant accumulator D‑0111 into the inlet of each compressor suction drum. Ideally, if the compressor anti-surge vapor flow and temperature control valve on quench liquid are perfectly matched and there is no time lag within the control system, the configuration of Figure 1 will work perfectly.

Unfortunately, if there is any mismatch, which in a real world situation always exists,   either too much quench liquid is added to the inlet stream whereby there is liquid level in the suction drums or not enough quench liquid is added whereby the discharge temperature keeps rising. In either mismatch case, the refrigerant compressor will shut down either from high liquid level in the suction drum or from high discharge temperature caused by the high inlet temperature due to recycle of hot gas. 

Surge Protection with Sparged Hot Gas using No Quench Liquid

A well-proven alternative system is shown in Figure 2 [3, 4] in which hot propane vapors from the compressor discharge, as controlled by the anti-surge system, flow through the sparged line submerged in the liquid filled portions of the HP (D-0116) and LP (D-0115) economizers, and under the tube bundle of the first stage chillers (E-0107, E-0108, and E-0111). There is no need for quench liquid as the hot vapors provide the heat to the available liquid propane in these services to generate the required propane vapors to prevent the compressor surge conditions.

”]The configuration of Figure 2 builds upon the recognition that:

• The shortage of propane refrigerant entering the compressor stages is caused by reduction in heat exchanger duty from changes on the process side of the chillers (E-0107, E-0108 or E-0111). This reduction of heat transfer duty is offset by routing the required amount of hot propane vapor from compressor discharge directly to the kettle side of the chiller to supplement process thermal requirements.
• The compressor suction drums are provided to prevent any carried-over liquid from entering the compressor and should essentially have no liquid in them.
• By diverting the anti-surge hot vapor away from the compressor suction drums assures that the compressor suction drums stay dry.
• By using the already present liquid refrigerant in the chillers and economizers it avoids any potential mismatch between the hot propane vapors and the quench liquid that may inadvertently buildup high liquid level and end up tripping the refrigerant compressor.

Conclusion

While both refrigerant compressor surge control systems are proven and used by the industry, the hot sparged gas with no quench liquid configuration of Figure 2 is more forgiving and reliable.

To learn more, we suggest attending our G40 (Process/Facility Fundamentals), G4 (Gas Conditioning and Processing), G5 (Gas Conditioning and Processing-Special), and PF81 (CO₂ Surface Facilities), PF4 (Oil Production and Processing Facilities), courses.

            John M. Campbell Consulting (JMCC) offers consulting expertise on this subject and many others. For more information about the services JMCC provides, visit our website at www.jmcampbellconsulting.com, or email us at consulting@jmcampbell.com. 

 By: Yuv R. Mehra

References:

1. Al-Shahrani, Saleh M. and Mehra, Yuv R., “Start-up Experience for Value Recovery from CCR Net Gas at Yanbu’ Refinery,” Advances in Hydroprocessing II Session, Paper 26d, 2007 Spring National Meeting of AIChE, Houston, Texas, USA, April 22-26, 2007.
2. Al-Shahrani, Saleh M. and Mehra, Yuv R., “Saudi Aramco installs new LPG recovery unit at Yanbu’ Refinery,” Oil & Gas Journal, Vol. 105.21, Jun. 4, 2007, p. 60.
3. Mehra, Yuv R., “Focus on Value-Chain Contributions during Process Technology Selection,” Worldwide Developments Forum, 86^th Annual Meeting of the Gas Processors Association, San Antonio, Texas, USA, March 11-14, 2007.
4. Mehra, Yuv R., “Focus on Value-Chain Contributions during Process Technology Selection,” Saudi Aramco Journal of Technology, Summer 2007, p. 2.