from The McNally Institute
The seal gland is that part that holds the stationary half of the mechanical seal and attaches to the stuffing box. The most universally used gland is labeled the American Petroleum Institute (API) gland:
The API (American Petroleum Institute) gland incorporates several important environmental controls and safety features. It should be specified for all petroleum refinery service because of the danger of an explosion or fire, but should also be recommended any time you are pumping any type of dangerous product. The gland has several features you need to understand:
- (DB) describes a non-sparking disaster bushing installed in the rear of the gland to prevent metal to metal contact in the event of a bearing failure in the pump. The maximum clearance of this bushing should be 0.025″ (0,6 mm) on the diameter. Carbon is the most common material used for this bushing. Please note that the bushing is pushed against a shoulder machined into the gland. A straight through bore with a snap ring retainer will not meet the API specification.
- (Q) is the quench, or drain and vent connection
- (F) Is the flush connection that can be connected to the discharge side of the pump for discharge recirculation, the suction of the pump for suction recirculation, or to an outside flushing connection to introduce clean fluid into the pump stuffing box.
Let’s take a closer look at some of these features and see where we might take advantage of them outside of the petroleum industry:
The Disaster Bushing
- Although the gland is expensive this bushing is an excellent feature any time you do a packing conversion because, in the event of a bearing failure, the shaft will deflect into the disaster bushing in the gland preventing a major destruction of the mechanical seal. Some manufacturers build a gland that has these features but does not meet the API specification for fitting size, bushing retention method, etc. These glands are commonly used outside the petroleum industry where the features are needed, but the cost is too high for the API standard version.
- The bushing prevents a massive leakage out the back of the gland when you experience a sudden seal failure. The majority of the leakage is directed to the drain connection. This is an important safety feature for any people in the area.
- Sometimes Teflon is substituted as a disaster bushing material for those low specific gravity products that freeze when released to atmosphere. The Teflon shrinks when it gets cold and provides a better sealing surface.
The Quench, or Drain and Vent connection
- This fitting allows you to bleed small quantities of low pressure steam between the seal and the disaster bushing to keep the seal faces warm when the pump is at rest. This is very necessary with those products that become viscous or setup and harden when cooled to ambient temperature
- In the event of a massive seal failure, most of the leaking fluid will be directed down the drain connection. Low specific gravity gases can be directed up the vent to a flare where they can be burned off
- Low-pressure steam can be connected to this fitting and a solenoid-operated valve can open in the event of a fire, to quench the fire with steam, and put it out.
- Discharge recirculation is used to direct fluid from the discharge side of he pump to the stuffing box
- This feature can be used to pressurize the stuffing box and prevent a fluid from vaporizing. This is commonly done in hot water applications. You should use a close fitting bushing in the end of the stuffing box to reduce the amount of pressure drop.
- Suction recirculation is used to direct fluid from the bottom of the pump stuffing box back to the pump suction
- We do this to clean up the liquid in the stuffing box. The fluid is being directed from in back of the impeller (where it has been centrifuged), into the stuffing box, and finally back to the pump suction.
- Use this fitting to vent the pump stuffing box when the pump is installed in a vertical position. Failure to do this can cause the seal faces to run dry
- Flushing is used to introduce clean fluid into the stuffing box at a pressure of about one atmosphere (14.7 psi or 1 bar) above stuffing box pressure.
- We like to use this connection to flush away undesirable fluid (solids, crystallizing, dangerous, etc.)