3.5.12 Student Book © 2004 Propane Education & Research CouncilPage 1 3.5.12 Reducing Cargo Tank Vapor Pressure Using a Plant Compressor In order to use

3.5.12 Student Book © 2004 Propane Education & Research Council Page 1

3.5.12Reducing Cargo Tank Vapor

Pressure Using a Plant Compressor

In order to use compressor to unload transports and recover the vapor left in the cargo tank, it is important to understand the process of reducing vapor pressure.

In this module you will identify:

(1) U.S. DOT regulations pertaining to unloading operations and attendance requirements

(2) Reasons for reducing cargo tank pressure

(3) Procedures for recovering propane vapor using the bulk plant compressor


DOT Regulations Pertaining to Unloading Operations

U.S. Department of Transportation regulations require that qualified persons be “in attendance” throughout the process of loading a cargo tank motor vehicle.

The person who is responsible for loading the cargo tank is also responsible for ensuring that it is so attended.

A person is “qualified” if he/she

• has been made aware of the nature of the hazardous material• has been instructed on the procedures to be followed in

emergencies

• is authorized to move the cargo tank

• has the means to do so .

3.5.12 Student Book © 2004 Propane Education & Research Council Page 1 & 2


3. The qualified person attending the unloading operation must remain within 25 feet of the cargo tank when the internal self-closing stop valve is open.

4. The qualified person attending the unloading operation must have an unobstructed view of the cargo tank and delivery hose to the maximum extent practicable, except during short periods when it is necessary to activate controls or monitor the receiving container.

5. When unloading the transport cargo tank using the plant compressor, the vehicle engine must not be running during the unloading operation.



6. The cargo tank must be equipped with a passive emergency discharge control system no later than the cargo tank’s first pressure test scheduled after July 1, 2001. After that date it must have a means to automatically shut off the flow of product without the need for human intervention within 20 seconds of an unintentional release caused by a complete separation of a liquid delivery hose (passive shut-down capability).


Reasons for Reducing Cargo Tank Pressure

Tank Car or Cargo Tank

Pressure, psig

33,000 Gal. Cap Tank Car—Gal. of

LP-gas

10,600 Gal. Cap Cargo Tank—Gal. of LP-gas

175 1170 375

150 970 310

125 770 246

100 570 182

75 370 118

50 170 54

Figure 1. Vapor Left in a Railroad Tank Car or Transport Cargo Tank Expressed in Gallons of Liquid LP-Gas



Figure 2. Propane Evacuation Time for a 33,000 Water Gallon Tank Car



It is not economical, however, to operate the compressor for the time required to recover the maximum possible amount of propane. To do so requires expense of unloading attendant time, electricity, and wear and tear on the compressor.

More than half of economically recoverable vapor is removed in the first hour of compressor operation. For that reason, many common carriers and propane marketers who operate company-owned transports, limit the vapor recovery time.


Vapor Recovery Procedures

Plant vapor compressors are used in unloading operations for the following reasons:

1. It saves time and eliminates the hazards associated with using the transport unloading pump.

2. It allows for recovery of some of the propane that remains in cargo tanks after the liquid has been unloaded.

3. It prepares the cargo tank for inspections, maintenance, or repairs.



Step 1: Position the transport at the unloading bulkhead connected to a compressor.

Step 2: Examine the plant piping and flow control valves to determine how vapor will move from the cargo tank to the suction side of the compressor and into the liquid section of the bulk storage tanks.



Figure 3. Typical Plant Compressor



LIQ U ID TRAP(SUCTIO N)

CARG O TANK

STO RAG ETANK

CO M PRE SSO RDISC HARG E

Figure 4. 4-Way ValveVapor Flow—Position One



Figure 5. Compressor and Bulk Plant Connections and Valves



Figures 6 & 7. 4-Way Valve & Piping Valve Positions

Operation of Valves for

Cargo Tank Unloading

4-Way Valve

Position Two

Valve A

Closed

Valve B

Open

Valve C

Open

3.5.12 Student Book © 2004 Propane Education & Research Council Page 5 - 7


Step 3: Check the Liquid Level Gauges on the Storage Tank(s).

Step 4: Connect the Bulkhead Vapor Hose to the Cargo Tank (if not already connected for liquid unloading).

Step 5: Inspect the compressor and check the crankcase oil level in the sight glass or by using the dipstick.

Step 6: Open the vent on the compressor’s liquid trap, if so equipped, or if not, verify that liquid is not present in the vapor hose and suction line.



Step 7: Verify that the 4-way valve is in the proper position for unloading the cargo tank. Check all in-line valves and bulk tank valves for proper open or closed position.

Step 8: Ensure that all shutoff valves are open in the vapor hose and cargo tank connection.

Step 9: Start the compressor in accordance with manufacturer's instructions.



Step 10: Closely monitor the cargo tank, compressor and receiving tank(s), ready to stop the compressor if any abnormal operating condition is noticed. Stop the compressor when the cargo tank pressure is reduced to the desired reading or the time allowed for vapor recovery has elapsed

Step 11: Close all valves in the transfer hose and on the cargo tank.

Step 12: Close all valves in both the suction and discharge vapor piping circuits. Close the ESV(s) at the transfer bulkhead, and the appropriate liquid and vapor valves in the bulk storage tank(s), if applicable.

3.5.12 Student Book © 2004 Propane Education & Research Council Page 7 & 8


Step 13: Disconnect the Transfer Hose and Prepare the Transport for Travel.

If for any reason the transfer operation is interrupted, or the attendant must leave the area, the compressor must be shutdown and transfer hoses disconnected as outlined in step 13. DO NOT leave transfer hoses connected to a cargo tank while unattended by a qualified person.

3.5.12 Student Book © 2004 Propane Education & Research Council Pages 9 - 12

Time to See If You Got the Key Points of This Module…

• Complete the Review on page 9 & 10.

• See if you are ready for the Certification Exam by checking off the performance criteria on page 11 & 12.

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3.5.12 Student Book © 2004 Propane Education & Research CouncilPage 1 3.5.12 Reducing Cargo Tank Vapor Pressure Using a Plant Compressor In order to use