SUBJECT : Let's clear up the confusion about flushing seals 3-6
Consumers use the term "flushing" to describe six different methods of bringing fluid to the stuffing box area of a centrifugal pump. Experienced seal people use different terms to differentiate between the methods.
A line is connected between the discharge of the pump and the stuffing box. The high pressure fluid is then recirculated through the stuffing box to the back of the impeller and eventually to the pump discharge. This technique presents several problems for maintenance people:
The only legitimate use of this technique is to pressurize the stuffing box to prevent a liquid from vaporizing. 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 the solids can clog up the heat exchanger.
When this line is used to pressurize the stuffing box you should keep several additional things in mind:
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, but if none is available you can install one in the piping or a pipe flange if the piping is not thick enough to be drilled and tapped.
Stuffing box pressure is almost always higher than the suction pressure of the pump. Liquid from behind the impeller will be circulated through the stuffing box to the pump suction. This liquid has been centrifuged by the impeller and the result is that the liquid in the stuffing box is considerably cleaner than what you're pumping. In many cases you can eliminate the need for bringing in clean liquid and diluting your product.
This environment control works very well in closed impeller pump designs and those open impeller designs that adjust towards the pump volute rather than the back plate, such as the Duriron (Flowserve) pump.
A clean liquid, from an outside source is brought into the stuffing box through a regulating valve at one atmosphere (15 psi. /1 bar) higher than stuffing box pressure. The liquid should be brought in at the bottom of the stuffing box to insure thorough cleaning. All of this liquid will eventually go into your product.
If you're using balanced o-ring seals you'll only need enough liquid to remove solids that might interfere with the seal movement. you'll not need additional liquid to provide cooling because balanced seals do not generate enough heat to cause problems in most applications.
Seal designs that have the springs out of the fluid require only one to two gallons per hour (4 to 8 Ltrs./ hour) of flush. NOTE: this is per hour, not per minute. If you're using designs with multiple springs in the fluid check with your manufacturer for his recommendations. The clean flush can come from several sources:
If you're using shop water as the flush you must be careful or solids in the flushing water will clog up the flow control valve. The shop water pressure also tends to vary through out the day and in some instances it can fall below the pump stuffing box pressure. Most states require an air gap in the line if you want to use shop or city water as a flushing medium. A back flow presenter valve is used many times but it is illegal in most states.
BARRIER OR BUFFER FLUID
Any time you use two seals in an application you'll need a fluid between them. If the fluid between the seals is higher than stuffing box pressure we call it barrier fluid. If it is lower than stuffing box pressure we call it buffer fluid The liquid can be circulated either by forced circulation, a pumping ring or convection. The method that you'll use will be dictated by the pressure, pump speed and shaft size. All seal manufacturers have charts available to give you the correct guidelines.
If you elect to use a forced circulation system be sure to introduce the fluid into the bottom connection and out the top connection. This arrangement will insure that the space between the seals is vented and proper cooling will take place.
Forced circulation is the recommended method with all vertical shaft applications, although it is possible to offset the centering of the seal gland and get a small amount of pumping action as the liquid circulating in the seal changes its velocity at the convection tank connections. Check with your local distributor for an explanation of this principle.
Many of the latest seal designs utilize a built in pumping ring to enhance convection. This pumping arrangement is very necessary when ever oil is used as the barrier fluid. The following illustration shows a typical convection system that can be used with two balanced seals.
Water is one of the best barrier or buffer fluids because of its high specific heat and good conductivity. Petroleum oil is probably one of the worse because of its low specific heat and poor conductivity. Keep this in mind when you select a barrier or buffer fluid for your seals.
The type of seal you select will determine if the barrier fluid has to be kept higher or lower than the stuffing box pressure. Fluctuating pressures are normal in this business so you should select seals that balance in both directions to eliminate any problems that might be caused when the barrier fluid or system pressure varies.
Be sure to connect the convection tank or forced lubrication system so that the inlet is at the bottom of the double seal and the outlet discharges from the top of the seal. This arrangement will allow the seal to vent, and insure that the passages are full of liquid.
JACKETING FLUID (B)
High temperature pumps have a cooling/ heating jacket installed around the pump stuffing box. If a jacket hasn't been installed on your pump it can 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're sealing. Carbon is an excellent choice because unlike Teflon it doesn't change dimensions too much with a change in temperature.
A small amount of liquid or steam through the jacket can control the stuffing box to what ever temperature range you need. In some instances cool heat transfer oil is utilized. Keep in mind that this jacket is also providing cooling to the bearing case as well as the stuffing box.
Be sure the jacketing fluid is free from calcium (hard water) or any substance that can build a film on the inside of the jacket surface and restrict the heat transfer. A number of cleaners are available if you experience this problem. Condensate is a good jacketing fluid that presents few problems and is usually available.
QUENCHING - Often called the vent and drain (Q connection on an A.P.I. gland)
BARRIER OR BUFFER FLUID
QUENCHING OR VENT & DRAIN - plus the disaster bushing.
The rest of the world calls all of these techniques "FLUSHING". Try to acquire the habit of using the proper terminology so that you'll avoid confusion when you communicate with seal people and your fellow workers.
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