Hot Oil Surge Drum Gas Blanketing Flow Rate Calc

1. Purpose

This procedure identifies the equations and data required to calculate the maximum gas blanketing rate to, and vent gas from, a Hot Oil Surge Drum.

2. General

The liquid circulation through a Hot Oil Surge Drum is closed loop (no net system liquid into or out of the drum). Therefore, the method used to calculate the gas blanketing rate to a conventional surge drum (open loop with net system liquid into and out of the drum) MUST NOT be used for a Hot Oil Surge Drum. Instead the method used for a Hot Oil Surge Drum is based on the thermal expansion of hot oil at the heater design duty. The gas blanketing rate is then assumed to be equal to the thermal expansion rate.

3. Diagram

4. Procedure

4.1 Approximate the gas blanketing rate to a Hot Oil Surge Drum using the equation for a relief valve protecting heat exchangers, condensers, and coolers against thermal expansion of trapped liquids.

Use the spreadsheet “Hot Oil Drum ONLY Blanket Rate.xls”, located on the Server at .

The basic equation is:

where:

gpm = flow rate at flowing temperature, in gallons per minute

B = cubical expansion coefficient per degree Fahrenheit for the liquid at the expected flowing temperature.

Calculate “B” using the following equation:

Normally, do not override the tool calculated “B” based on only API gravity. Override the tool if “B” is available from the hot oil vendor. Below are typical values for hydrocarbon liquids and for water at 60 degrees Fahrenheit.

H = total heat transfer rate, BTU/h (use design heater duty)

G(ave) = average specific gravity between design heater inlet and outlet

C(ave) = average specific heat of liquid between heater inlet and outlet, BTU/lb/°F

4.2 Next, convert the flowing gpm to flowing gas volume units per hour using the equation:

• Then, convert the flowing gas volume to standard gas volume using the drum operating pressure and temperature. The gas volume converted to standard conditions is the maximum gas blanketing rate using the equation:

where:

T(opr) = drum operating temperature (°R)

P(opr) = drum operating pressure (PSIA)

Example Data

API = 28.5

B = 0.0004

H = 57,500,000 BTU/h

G(in) = 0.697

G(out) = 0.672

G(ave) =

C(in) =

C(out) =

C(ave) =

T(opr) = 462°F

P(opr) = 170 psig

6. Example Calculations

• Liquid Flow

• Liquid to Flowing Gas Volume (gpm to cfh)

• Flowing Gas Volume to Standard Conditions (cfh to scfh)