Compressor Sizing

Size, configuration and speed of compressors shall be selected on the basis of the maximum operational flow rates and pressures required. Compressors shall be sized within the maximum flow rating of the excess-flow valves installed on the incoming supply tanks when used for unloading shipments and on the plant's LPG tanks when used for loading shipments. When a compressor is used for transferring liquid LPG in this manner, the liquid transfer rate is less than the vapor displacement of the compressor. The compressor manufacturer shall be consulted to obtain the expected transfer rate under the plant operating conditions. Typical compressor volume flows and pressure differentials for operations in an LPG marketing terminal are as follows:

Volume Differential Pressure

25–340 std m3/h 70–140 kPa

Table 4.2.2: Compressor Volume Flows and Pressure Differentials Flow Rate: std m3/h at 15.6 °C and 101.325 kPa

4.2.2.1 Compressor Installation Requirements

Compressors shall be installed in a freely ventilated location. The compressor shall be securely mounted on a suitable foundation or base plate in accordance with the compressor manufacturer’s recommendations. The compressor casing shall not be subjected to excessive strains transmitted to it by the suction and discharge piping. Compressor piping shall be suitably braced to minimize vibration Properly sized knock out pots, fabricated to appropriate pressure vessel code, shall be installed on the inlet of the compressor to prevent liquid from entering the compressor.

On each knock out drum, a high level shutdown system is required so that if too much liquid accumulates in the drum, the compressor will automatically shutdown before liquid carry over to the compressor occurs. Check valve shall be installed on the discharge side of all centrifugal compressors. Strainers shall be installed on the suction piping. The strainer element shall be designed and installed so that it can be serviced.

The compressor shall be provided with the following operational and safety features:

1. A local start-stop pushbutton shall be installed in the vicinity of the compressor.

2. A remote Emergency Shutdown (ESS) pushbutton shall be provided in a safe location in case the local start-stop pushbutton is not accessible because of fire or vapor cloud.

Compressor shall be interlock to stop when any Emergency Shutdown System (ESS) pushbutton is activated if such a system is provided.

Figure 4.2.2: Knock-out drums (liquid traps)

Positive displacement compressors shall be equipped with suction and discharge shutoff valves. A blowdown valve which relieves the trapped pressure when the compressor is shut down shall be provided. Automatic blowdown valve is acceptable. Each positive displacement compressor shall be equipped with a pressure-relieving device on the discharge side discharging to a safe location. Following are installation requirements for compressors in LPG bulk plant service:

1. Compressors shall not be positioned underneath LPG tanks. They shall be located outside LPG drainage and impound areas and in accordance with the spacing requirements defined in the section of this guide entitled “Equipment Spacing to Maximize Separation” in Chapter 2. 2. The compressor shall be bolted and grouted to a substantial concrete

foundation in accordance with compressor manufacturer's recommendations. Often the compressor and auxiliary equipment are supplied on a steel frame, which is then bolted and grouted to the foundation. The proper procedures to mount, level, install and shim the compressor can be found in the API 686 “Recommended Practices for Machinery Installation and Installation Design.” Pipework to the compressor shall be fitted and supported in accordance with the manufacturer's recommendations so as not to subject compressor components to excessive stress.

3. On the suction side of the compressor appropriate means shall be provided to prevent liquid phase of LPG from entering the compressor.

This is typically done by installing a suction knock out drum (liquid

trap) with a high level shut down to stop the compressor driver. Traps

shall be fabricated to an approved pressure vessel code and in addition, equipped with an independent liquid level alarm switch and a liquid drain connection. For small compressors a knock out drum with a float to close the inlet line to the compressor may be provided. When the inlet valve is closed the compressor should trip, e.g. on high current or low suction pressure, prior to pulling vacuum.

4. Reciprocating compressors, since they are positive displacement machines, require a pressure relief valve on the discharge line. There shall be no valves in the line between the compressor outlet and relief valve. The pressure relief valve shall be sized for the maximum flow of the compressor. The valve may be routed to an appropriate, safe location.

5. Standard valving on the inlet and outlet of a compressor is as follows: + Remote and automatic shutdown valve at the tank inlet and

outlet.

+ Manual block valve at the compressor suction. + Manual block valve at the compressor discharge.

+ A valved connection on the discharge for a pressure gauge. This connection shall be two plane gusseted and seal welded in accordance with GP 3-18-1.

+ An optional four way valve to automatically reverse the suction and discharge when the compressor is to be used for both loading and unloading.

6. Suction and discharge piping shall be fabricated so that they exert minimum stress on the compressor. Guidelines for piping fit up can be found in GP 3-19-1. Incorporation of one or two horizontal ninety degree bends near the compressor may provide adequate flexibility but a piping stress analysis will reveal any significant problems. Suction piping shall also be designed without low points and pockets to trap liquids. For reference, GP 3-3-4 provides requirements for reciprocating compressor suction and discharge piping. Pipework for the transfer of liquid LPG shall be sized so the overall pressure drop in the system does not result in excessive condensation. Experience in typical LPG installations indicates that this pressure drop is generally not in excess of 200 kPa. Install suction and discharge lines so that any condensate that may form in the piping system does not drain into the compressor. Isolating valves shall be installed on either side of the compressor to permit its removal or maintenance while minimizing the volume of LPG vented to atmosphere.

7. In cold climates compressor suction lines in Butane service shall be specially reviewed to ensure condensed vapor cannot collect in the suction line. Sloping the line continuously back to the tank and avoiding low points may do this.