Pump Non-contacting Gas Seals

1. Purpose

This procedure describes the use and requirements for centrifugal pump non-contacting gas-lubricated seals.

2. General

Traditionally unpressurized dual (tandem) seals with buffer fluid from an API Piping Plan 52 have been used to seal centrifugal pumps handling a flashing or environmentally hazardous liquid. Since 1995, non-contacting gas lubricated seals have become widely used in the refining industry. The advantages of non-contacting gas seals over buffer fluid seals are:

• Zero product release to atmosphere
• Lower initial costs
• Elimination of API Plan 52 seal pot
• Lower operating costs
• Lower maintenance (extended seal life)
• Reduced energy costs
• Seal flush and seal flush coolers are eliminated

3. Non-contacting Gas Lubricated Seals

Non-contacting gas lubricated seals have two sets of rotating and stationary faces similar to liquid lubricated pressurized dual seals. A gas (typically nitrogen from the refinery nitrogen header) is injected between the two sets of seal faces. Similar to compressor dry gas seals, 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). Less than 0.1 SCFM of nitrogen leaks into the process.

A gas seal control panel is provided to filter and regulate the seal gas and monitor performance. The seal gas must be available at 30 psi above the maximum process suction pressure to ensure seal face lift-off upon pump start-up. A pressure regulating valve in the control panel maintains this pressure differential between the nitrogen and the process. The filter-separator provides an efficiency of 98% on liquids 1 micron or greater and solids 3 microns or greater.

A low-pressure alarm warns upon loss of the nitrogen supply and a high nitrogen flow alarm warns in the event of a seal leak.

Since the inner seal is lubricated by the clean, dry nitrogen, neither an inner seal flush nor throat bushing is required. There is no concern over process fluid flashing in the seal area.

Non-contacting gas lubricated seals may be specified up to an operating temperature of 500 F and up to an maximum suction pressure of 300 psig. (Note that the maximum suction pressure must be 30 psi below the normal nitrogen header pressure).

4. Specification

Inflection Point Engineering shall specify non-contacting gas lubricated seals where applicable (unless the customer takes exception in the Basic Engineering Design Questionnaire). Under seal type in the 501 specification, indicate "pressurized non-contacting." Do not indicate a seal flush or and auxiliary seal flush. Reference Note 2, which reads as follows:

“Pump vendor shall provide non-contacting pressurized dry gas seals. The barrier gas shall be nitrogen from the plant nitrogen header available at ___ psig. Pump vendor shall provide a seal control panel for each pump. The panel shall include the following:

• Coalescing filter (removal efficiency of 98% on liquids 1 micron or greater and solids 3 microns or greater)
• Pressure regulating valve
• Pressure gauge
• Flow meter low range
• Flow meter high range
• High-flow and low-pressure alarm switches, which shall activate a control center trouble alarm (by others). The contractor and pump vendor shall coordinate switch selection.

5. P&ID Detail

P&ID detail "DD" (dual dry) shall be referenced at the pump on the P&ID. This detail simply shows the nitrogen line with an isolation valve coming into the pump seal console, which is provided by the seal vendor.