DryGasSeals

1. Purpose

This procedure describes the use and requirements for centrifugal, axial and screw compressor dry gas seals.

2. Background

Dry gas seals have become the industry standard method of sealing centrifugal compressor flammable gas.

Dry gas seals offer the following advantages over oil seals:

• Elimination of sour oil disposal
• Elimination or reduction in size of the oil console
• No seal oil consumption
• Reduced process gas leakage
• No oil contamination of process
• Reduced energy costs

3. Dry Gas Seals

3.1 Dry gas seals have rotating and stationary faces similar to traditional oil seals. A gas (typically process gas from the compressor discharge) is injected between the primary seal face and the process side of the compressor, with most of this gas returning across a labyrinth seal to the process. Grooves on the rotating seal face pump the gas inward towards the non-grooved portion of the face that serves as a barrier to gas flow. This raises the pressure between the faces and separates them. Typical seal face separation is 0.0001 to 0.0002 inches (0.0025-0.0050 mm).

A seal gas console (on a separate skid from the lube oil console) is provided to filter and regulate the seal gas and monitor seal performance. Process gas is taken from the compressor discharge where it passes through one of two coalescing filters (one operating, one on standby) with stainless steel housing. The filter-separator provides an efficiency of 98% on liquids 1 micron or greater and solids 3 microns or greater. The filter has a high differential pressure alarm to warn if it is becoming plugged. The seal gas piping shall be stainless steel (or other metallurgy if required by process gas) and heat traced and insulated to avoid any condensation.

Controlling the temperature of the seal gas above the upstream separator temperature is critical to seal performance. If the temperature of the gas in the piping from the compressor discharge to the seal is allowed to drop below saturation temperature, liquids could form and damage the seal faces. This dry gas seal piping shall be routed from the compressor discharge piping to the seal inlet connections with no pockets or dead legs (including normally closed bypass valves). The length of piping shall be minimized.

The seal gas shall be on either automatic differential pressure control or automatic flow control per the seal gas console vendor’s requirements

3.2 Unpressurized dual dry gas cartridge type seals are specified. Clean, dry seal gas is injected on the process side of the primary seal faces and approximately 2-3 acfm (3.4-5.1 m3/hr) leaks across the seal into the barrier between the primary and secondary seals. Some of this seal gas leaks across the secondary seal while most of the gas is vented to flare or safe disposal. There is instrumentation to measure high flow or high pressure in the vent and to provide an alarm in the event there is excessive primary seal leakage. Upon a high-high leakage condition across the primary seal there is a 2 out of 3 voting type shutdown. There is also a low flow alarm in the primary seal vent to warn of excessive secondary seal leakage. (Alternate means of detecting secondary seal leakage such as a hydrocarbon analyzer are acceptable.)

Nitrogen is injected at approximately 2 psig (0.14 kg/cm2g) on the outboard side of the secondary seal to act as a separation between the process gas and the bearing box to prevent process gas from escaping to atmosphere or contaminating the lube oil system. High and low differential pressure alarms are provided between the nitrogen injection point and the secondary vent to warn if the separation gas pressure is too high or too low. Process gas and nitrogen are vented to atmosphere or safe disposal.

3.3 Alternately pressurized dual seals may be specified. A reliable clean gas source (typically nitrogen from the unit nitrogen header) is injected in the barrier between the seals at 20-30 psi above compressor suction pressure. Nitrogen bottles could be an on-line backup source of seal gas should the unit nitrogen header be temporarily disabled.

Typical applications for this type of seal are as follows:

• Platforming Recycle Gas Compressor
• FCC Gas Concentration Wet Gas Compressor
• Oleflex Reactor Effluent Compressor

To prevent liquid from accumulating at the process side of the seal face, a drain should be specified.

4. Startup Gas

During circuit pressurization (with compressor idle), startup and shutdown or unstable operation, an alternate clean seal gas should be supplied. This outside gas shall also be used for any condition when the seal supply from the compressor discharge is absent. Suitable provisions shall be made to avoid contamination, backflow, or overpressure of the outside seal gas supply system.

5. Project Specification 508 Note

5.1 The following note (16) should be referenced, for un-pressurized seals, in the 508 Project Specification:

Note 16. The contractor is responsible for proper installation of the dry gas seal system per Inflection Point Engineering Standard Specification 5-15 Section 2.3. The dry gas seal piping shall be heat traced and insulated from the compressor discharge to the connections into the seals. Also, there should be no pockets or dead legs (including normally closed bypass valves) and length of piping should be minimized.

5.2 If pressurized dual seals are used replace Note 16 in the 508 project specification with the following alternate:

Note 16. Alternate: The contractor is responsible for the proper installation of a dry gas seal system. The compressor vendor shall offer doubly opposed dry gas seals per API 614 4th Edition Figure 4A-1 including the optional flow element. Seal(s) shall be cartridge type. The dry gas seal module shall be on a separate skid from the lube oil console. There should be no pockets or dead legs (including normally closed bypass valves).

The seal barrier gas shall be nitrogen from the plant nitrogen system. The seal vendor shall advise the required dew point and minimum supply pressure required. The Contractor shall provide bottled nitrogen to automatically engage on loss of nitrogen header pressure in order to provide uninterrupted seal barrier gas.

The seal vendor shall provide a drain plug between the process side labyrinth seal and the process side dry gas seal; faces to drain any liquid that could collect. This drain will be used instead of the buffer gas supply line per API 614 4th Edition Figure 4A-1. The drain line (by contractor) shall include a level transmitter control-type trap and shall be routed to the closed drain header.

6. Standard Note

Standard Specification 5-15 has the following note:

6.1 Shaft Seals

a. Shaft seals for process gas compressors shall be self acting gas type.

b. The compressor vendor shall offer tandem (unpressurized dual) dry gas seals for the compressor casing. Seals shall be cartridge type. The dry gas seal module shall be on a separate skid from the lube oil console. The seal model shall be a selection of duplicate size and design which has a successful record of field experience. The seal gas shall be automatic differential pressure controlled gas from the compressor discharge. (Automatic flow control is also acceptable if preferred by the seal vendor for a specific application).

A nitrogen separation gas shall be provided to prevent flow of bearing lubrication oil to the seals.

The entire dry gas seal system piping from the compressor discharge piping to the connection into the seals shall include stainless steel piping insulated and heat traced by others to prevent any condensation. The heat tracing objective temperature shall be equal to or greater than the normal operating temperature of the upstream separator.

Coalescing filters shall have stainless steel housing and a removal efficiency of at least 98% on liquids 1 micron or greater and solids 3 microns or greater. Transfer valves shall have stainless steel bodies with stainless steel internals.

Dry gas seal piping shall be routed from the compressor discharge piping to the seal inlet connections with no pockets or dead legs (including normally closed bypass valves). The length of piping shall be minimized.

The following alarms and shutdowns shall be provided:

*Alternate means to provide on-line monitoring of the integrity of the secondary seal, such as a hydrocarbon analyzer, are acceptable.

c. The collapse pressure of the filter elements shall be minimum of 50 psi (3.5 kg/cm²) differential.

d. Primary seal vents shall be routed to flare or safe disposal.

e. Testing

(1) Operating and spare seals shall have the following shop tests:

(a) Gas leak at a pressure equal to the settling out pressure with a mole weight gas equal or less than the seal gas.

(b) Functional (Run for minimum of one hour at operating pressure and speed.) Following the test, the seal elements shall be examined for wear and general condition.

(2) Control system shall be tested for cleanliness (No discoloration or hard particles found on 20 mesh screen after the module is blown for 5 minutes with 100 psig (7.0 kg/cm²(g)) dry, filtered gas.)

(3) The job dry gas seal shall also be used during the main equipment's mechanical run test.

f. Provide a plugged purge connection for secondary gas seal supply for future use. Provide a labyrinth between the primary and secondary seals to isolate the primary seal gas from the secondary seal. This secondary gas seal supply will prevent process gas from leaking to the atmosphere and will also purge the cavity between the two seals to prevent any liquid build-up.

g. Provide spare seal cartridges for the entire compressor.

h. Normally during compressor operation, the seal gas shall be supplied from the compressor discharge.  In addition, an outside supply of seal gas shall be provided to maintain seal cleanliness during periods when the compressor is pressurized but not operating or during startup when the process is unstable.  This outside gas shall also be used for any condition when the seal supply from the compressor discharge is absent. Suitable provisions shall be made to avoid contamination, backflow, or overpressure of the outside seal gas supply system.

i. See alarm and shutdown notes on sheet 9 of Project Specification 508 and the following API 614 4th Edition Figures: