Practices for Controlling Pump and Compressor Fugitive Emissions

1. Purpose

This procedure provides information on current Inflection Point Engineering practices for controlling fugitive emissions for pumps and compressors. Inflection Point Engineering designs follow current industry practices. This procedure also provides alternative designs if local environmental protection agencies require stricter control of fugitive emissions than the current Inflection Point Engineering practice allows.

2. General

2.1 Centrifugal Pumps

Inflection Point Engineering’s current approach is to specify single mechanical seals, with dual seal retrofit capability for most services. Dual seal systems are specified for some severe services such as hot phenol, HF acid, low specific gravity hydrocarbons (approx. 0.5 sp. gr.), hydraulic power recovery turbines and certain carcinogenic compounds. See Procedure ."

When unpressurized dual seals are used the pot between the two seals is vented to a closed vent system, typically the flare header. However, there is still a small escape of process fluid across the outer seal face to the atmosphere, usually under 10 ppm.

If lower fugitive emission levels are required use one of the following zero fugitive emission alternatives:

a. Sealless Pumps

Sealless pumps have zero leakage, therefore zero fugitive emissions. Note that sealless pumps have a limited range of head, flow and horsepower. See BEDQ Item 5.6.d for sealless pump limitations.

b. Pressurized Dual Seals

Pressurized dual seals do not leak process fluid to the atmosphere and therefore have zero fugitive emissions. The barrier fluid leaks into the process. Whenever pressurized dual seals are specified confirm that the barrier fluid is compatible with the process.

c. Dry Non-Contacting Gas Seals

See Procedure ."

2.2 Centrifugal Compressors

Inflection Point Engineering specifies tandem dry gas seals with a primary vent to flare for process gas compressors. Between 0.2 and 2 cfm process gas, per seal, leaks through the primary vent to flare. The current practice allows for less than 0.5 cfm of process gas to leak through a secondary vent to atmosphere outside the compressor shelter. See ."

The Inflection Point Engineering Centrifugal Compressor Specification also calls for a labyrinth between the primary and secondary seals. The use of the secondary seal gas is left up to the user if zero fugitive emissions are desired.

If a secondary seal gas, typically nitrogen, is injected between the labyrinth and the secondary seal, the primary seal gas will be isolated from the secondary seal and no process gas will leak to the atmosphere.

In some cases where the process suction pressure is below the Nitrogen header pressure, Inflection Point Engineering specified pressurized dual dry gas seals. No process gas will leak to the atmosphere.

2.3 Reciprocating Compressors

Reciprocating compressors use a packing box with several sets of rings to seal the translating rod against the stationary housing. Packing, however, is not a zero leakage seal.

Inflection Point Engineering current design calls for an auxiliary packing arrangement with the addition of at least two packing rings on the distance piece side of the packing. A nitrogen panel with stainless steel back plate and low nitrogen pressure alarm is provided as a buffer to the auxiliary packing. This is designed to contain all process gas within the packing. The nitrogen source pressure requirement is approximately 10 to 15 psi greater than the packing vent pressure. Estimated consumption is 5 SCFM.

Inflection Point Engineering calls for a TI on the packing vent/drain line to warn of excessive packing leakage. Since the vent is open to the flare, the pressure may never build up when the packing leaks. However, the vent/drain line will increase in temperature as a higher quantity of hot process gas passes through the line.