OSHA 1910 SEALING REGULATIONS O016
The Process Safety Management Standard was created to prevent the unwanted releases of hazardous chemicals. The standard identifies more than 130 specific toxic and reactive chemicals covered in specific quantities and processes that involve flammable liquids and gases in quantities of 10,000 pounds or more. At this writing, hydrocarbon fuels may be excluded if used solely as a fuel.
A process is covered if it involves toxic or reactive, highly hazardous chemicals at or above the specified threshold quantity of the standard. The threshold quantity is the amount of the chemical present at any given point in time, not aggregated over a period of time.
The clean air act was created in 1990 to address the escape to atmosphere of Volatile Organic Compounds (VOC). The proposed amendments for chemical plants apply to any component in contact with a substance that is at least ten percent applicable VOC, and is in gaseous or light liquid VOC service more than 300 hour annually.
To determine the amount of VOC in a gaseous leak, the VOC is measured at a distance no more than one centimeter (less than a half inch) from the source.
- Phase 1 at the onset ………………. greater than 10000 ppm.
- Phase 2 one year later ………………. greater than 5000 ppm.
- Phase 3 two and one half years later…. greater than 1000 ppm.
- Polymerizing polymer greater than 5000 ppm.
- Food/ Medical greater than 2000 ppm.
- All other pumps greater than 1000 ppm.
For components with moving parts (pumps and mixers) the first attempt to repair a leak must be made within two days after the leak is detected. The standard also requires the monthly visual inspection of all single mechanical seals.
Pumps with dual mechanical seals can be exempted from the monthly inspection if the barrier fluid pressure between the seals is at a higher pressure than the pump stuffing box pressure at all times and the barrier fluid is not a light liquid VHAP (Volatile hazardous air pollutant), or is equipped with one of the following three features designed to prevent VOC emissions from the outboard seal:
- A barrier degassing reservoir that transports the gas or vapor to a VOC control apparatus. That system prevents applicable VOC from accumulating where it can be emitted from the outboard seal.
- A closed-loop system that purges the barrier fluid into a process stream and returns process fluid to the process without venting to the atmosphere.
- A sensor that detects failure of the seal system, the barrier fluid system or both.
If a leak is detected between the seals, the first attempt at repair must be no later than five days and the repair or replacement no later than 15 days.
Single seals are available that can satisfy current standards. They must be monitored monthly (EPA Method 21) and visually inspected weekly. If they are detected leaking:
- The first attempt at repair must take place within five days.
- Repair or replacement within fifteen days.
- Phase three pumps, when the leak is greater than 2000 ppm.
The only sensible approach to the sealing of fluids and gases identified in these acts is the use of dual seals designed with a two way hydraulic balance and the barrier fluid pressurized at least one atmosphere above maximum stuffing box pressure. The tandem configuration would be a logical choice for both rotating and stationary versions of a dual seal.
|In this diagram I am showing a simple, unbalanced version of a dual tandem seal for demonstration purposes.|
The preferred configuration for the dual seal would be:
- Hydraulically balanced seals with the inner seal balanced in two directions (two-way balance.)
- The stationary version of the tandem configuration is the most desirable.
- A pressurized convection tank connected between the seals.
- A remote indication of the pressure in the convection tank.
- A pumping ring built into the seal to increase the circulation between the seals.
- A cartridge version of the seal, with some sort of “self aligning” feature to prevent excessive face movement of the stationary seals.
- Dual hydrodynamic and hydrostatic gas seals are becoming very popular in these applications.
The preferred arrangement should not only satisfy the regulations, but also will provide additional safety features:
- The seal will not blow open if the barrier fluid pressure is lost. The two way balance will insure that the inner seal faces will stay closed when the pressure reverses.
- The seal will be less sensitive to solids in the fluid. Centrifugal force will work for you throwing the heavier solids away from the lapped faces.
- The higher barrier fluid pressure will help to lubricate the seal faces in some gas applications.
- You can easily detect if either of the seals fails prematurely. The convection tank pressure will either drop to system or atmospheric pressure depending upon which seal wears out or fails first.
- The higher barrier fluid pressure will prevent some fluids such as ethylene oxide from penetrating into and destroying the dynamic elastomer in the inboard seal.
- The barrier fluid can prevent the formation of ice outboard the seal in some low specific gravity applications.