API plans that I use. 15-05
The American Petroleum Institute (API) issues guide lines to help petroleum people select and then pipe various types of controls for mechanical sealing applications. These piping arrangements are described in a series of plans issued by the API.
Although 17 plans are described, only a few are really needed. Any time you have 17 choices there is bound to be some confusion. In the following paragraphs I will describe those API plans that I use on a regular business. In the following paragraphs I will describe where I use these plans and, in the process, hopefully simplify your selection decisions
Plan #01 Discharge recirculation to pressurize the pump stuffing box
A recirculation line is connected from the discharge side of the pump to the stuffing box. The high-pressure discharge fluid is then recirculated through the stuffing box to the rear of the impeller and eventually to the pump discharge.
This technique presents a couple of problems for maintenance people:
The only legitimate use of this discharge recirculation line is to pressurize the stuffing box to prevent a liquid from vaporizing, and that is where I recommend its use.
Be careful if you use this method in hot water applications especially if a heat exchanger is installed in the recirculation line. A high temperature water or steam leak in any of the fittings could be dangerous for any personnel in the area and any entrained solids can clog up the heat exchanger.
When this line is used to pressurize the stuffing box you should keep several additional thoughts in mind:
Plan #02 Circulation through a pump heating or cooling jacket
Condensate, steam, or heat transfer oils work well as the heat transfer medium
This is the best method of controlling the temperature of a fluid in the pump stuffing box if the temperature must be controlled when the pump is not running.
There should be no recirculation or flushing lines attached to the stuffing box when this environmental control is in use. Some of these lines could be covered up by insulation, so look out for them. High temperature pumps should come equipped with a cooling or heating jacket installed around the pump stuffing box. If a jacket (B) has not been installed on your pump it can probably be purchased from the pump manufacturer or an "after market" supplier.
The secret to using a jacketed stuffing box is to install a thermal bushing into the bottom of the stuffing box and then "dead end" the stuffing box liquid. Dead ending means that no suction or discharge recirculation lines should be installed. Any material that has poor thermal-conducting properties will be satisfactory for the bushing provided it is compatible with what you are sealing. Carbon is an excellent choice because unlike Teflon® it does not change dimensions too much with a change in temperature.
Plan #13 Suction recirculation to keep a flow going through the stuffing box
This is the best general piping layout for most of your applications
In this arrangement a line is connected between the suction of the pump and the bottom of the stuffing box or seal gland connection. Many pumps have a connection already tapped at the suction throat of the pump for a suction gage. If this fitting is available you can install one in the piping or in the pipe flange if the piping is not thick enough to be drilled and tapped.
Try to make the stuffing box connection as close to the seal faces as possible to insure a good circulation through the stuffing box.
Plan #62 The quench gland. Often called the API gland
In general usage the term quench is frequently used to describe the cooling of a heated metal, or to extinguish a flame. In the seal business, quench (Q) is a term used to describe the introduction of a fluid outside the mechanical seal. We use this fluid to:
Plan #53 Barrier/ buffer fluid using an internal pumping ring
Any time you use dual seals (two seals) in an application; you should have a fluid circulating between them to prevent the generation of unwanted heat. The following illustration shows a tandem arrangement of dual rotating seals.
The following illustration shows a typical convection system that can be used with two balanced seals. Check with your supplier for recommended pipe size, height and length. CAUTION! Do not hang the convection tank on the seal gland. The increased offset weight can distort the internal clearances of the gland.
Plan #3. Flushing to remove unwanted fluid in the stuffing box.
Do not confuse flushing fluid with discharge recirculation, suction recirculation, quenching, barrier fluid, buffer fluid or jacketing fluid.
Flushing has a very specific meaning:
When you should use flushing plan #32?
Here are some of my comments about each of the plans recommended by the API:
Plan 01. A line from the pump discharge is connected to the pump stuffing box. We discussed this one as a legitimate method of raising stuffing box pressure
Plan 02. The stuffing box is dead-ended. Heating or cooling fluid is circulated through the stuffing box jacket. This is the best method of controlling thew stuffing box temperature when the pump is not running.
Plan 11. A line is connected from the discharge side of the pump and recirculated through an orifice into the gland flush connection. Orifices are hard to size and since many pumping fluids contain solids orifices are easy to clog.
Plan 12. A line is connected from the discharge side of the pump and recirculated through a strainer and control orifice to the gland flush connection. If solids are present in the fluid the strainer will frequently clog along with the orifice
Plan 13. A line is connected from the bottom of the stuffing box, through a flow control orifice, to the suction piping. Controlling the size of the orifice to get exactly the right flow is difficult. In most cases you will not need the orifice
Plan 21. Discharge recirculation through a flow control orifice and cooler into the seal chamber. You are cooling with high-pressure, pump discharge fluid. Not too good an idea! Hot, high-pressure fluids can be dangerous if the cooler or its inlet and outlet-line fittings leak. There are better cooling methods that utilize lower fluid pressure. Jacketing, barrier fluid and quenching come to mind. Also note that with this arrangement you only get the cooling effect when the pump is running. This could cause a premature seal failure when the pump stops.
Plan 22. Discharge recirculation is passed through a strainer, orifice and then through a cooler to the stuffing box. The above mentioned problems with the cooler, orifice and strainer can combine together for a real problem
Plan 23. A pumping ring is installed in either the stuffing box or within the mechanical seal components that will pump the stuffing box fluid through an external cooler and then back to the stuffing box. This is a good arrangement because it uses lower pressure cooling fluid. You find this arrangement on the stuffing box of many boiler feed pump sealing applications.
Plan 31. Discharge recirculation through a cyclone separator to the stuffing box, Cyclone separators are not very effective in removing the solids that fail mechanical seals. I would not waste my time with this one.
Plan 32. Flushing liquid from an external higher pressure source to the stuffing box. A good solution if you can tolerate some product dilution and insure the flushing pressure remains higher than the stuffing box pressure.
Plan 41. Discharge recirculation through a cyclone separator and cooler to the stuffing box. This combines two problems into a bigger problem.
Plan 51. An external reservoir providing a dead ended blanket of fluid to the quench connection of the API gland
Plan 52. External reservoir providing buffer fluid for the outer seal of an un-pressurized dual seal arrangement. During operation an internal pumping ring provides circulation. The reservoir is connected to a vapor recovery system and is maintained at a pressure less than the pressure in the seal chamber. This is a common and good environmental control. If the inner mechanical seal is hydraulically balanced in both directions you can use this arrangement most of the time.
Plan 53. Pressurized external barrier fluid reservoir supplying clean fluid to the seal chamber. Circulation is by an internal pumping ring. Reservoir pressure is greater than the process pressure being sealed. Typically used with a tandem dual seal. Likewise a good environmental control when the pumping fluid is dangerous.
Plan 54. Pressurized external barrier fluid reservoir or system supplying clean fluid to the seal chamber. Circulation is by an external pump or pressure system. Reservoir pressure is greater than system pressure being sealed. Typically used with tandem dual seal. Another good environmental control
Plan 61. Tapped connect for purchaser's use. (flush connection). We discussed flush a little earlier
Plan 62. External fluid source providing a quench (steam is the most common quench fluid). Typically used with a close fitting bushing in the rear of the gland to prevent the steam from entering into the bearing cavity. A good environmental control. We discussed it earlier
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