HEAT GENERATION AT THE SEAL FACES ST 005
The numbers shown in the following chart are what you might find in the stuffing box of a centrifugal pump. The following calculations will compute the heat generated at the seal faces if you are using an unbalanced mechanical seal of the type supplied by most pump manufacturers.
We will make our first calculation in the inch size:
Hydraulic closing force = 100 lbs/in2 x 1 in2 = 100 lbs
Hydraulic Opening force = An average of 50 psi on the faces x 1 in2 = 50 lbs.
100 lbs closing + 30 lbs spring force – 50 lbs opening = 80 lbs closing80 lbs x 0.2 x 1885 fpm = 30160 Ft lbs./ min
778 ft lbs. / min. = 1 Btu.
30160 / 778 = 38.8 Btu. /min.
38.8 Btu. / min would raise 1 pint of water 38.8 degrees Fahrenheit each minute, so we would have to flush in 38.8 pints (4.84 gallons per minute) of cooling water if we did not want the product to get hot.
Metric looks like this:
65 Kg closing – 32,5 Kg. opening + 13 Kg. Spring pressure = 45.5 Kg. closing45.5 x 0.2 x 455.5 = 4145.3 Kg Met. /Min.
A Newton Meter is a Joule so we have 690 Joules/ sec.
690 Joules/Sec. x 60 Sec./Min. = 41,400 Joules per minute.41,400 x 0.239 joules per calorie = 9,895 calories (9.9 Kilocalories) per minute.
9.9 Kilocalories per minute would raise 9.9 liters of water one degree Centigrade per minute.
Since we have only one half a liter (500 cc) in the stuffing box, we would have to flush in 9.9 x 2 or 19.8 liters / minute to prevent a temperature rise in the stuffing box.
The good news is that the amount of heat generated by a properly installed balanced mechanical seal is insignificant.
The amount of heat generated by packing varies with the type of packing and the individual packing the pump. On the average you will find that packing generates six times the heat of a balanced mechanical seal.
In addition to the heat generated by the pressure in the stuffing box we have the heat caused by too much spring compression on the lapped faces. There are lots of chances to make an installation error:
- No print was used during the assembly, or the mechanic cannot read the print he was given.
- The shaft was marked in the wrong location.
- The mechanic used the wrong marking tool. The mark is too wide.
- The sleeve moved when the impeller was tightened.
- The impeller was adjusted after the seal was installed.
- A cartridge seal was installed on the shaft, by pushing on the gland. Interference from the sleeve elastomer has caused an over compression of the seal. In some dual seal applications the outer seal will become under compressed.
- The shaft moved because of thrust. Above 65% of a pump’s efficiency the shaft moves towards the volute. This will compress the seal faces if you have an outside mounted non-metallic seal. In some dual seal designs the inner faces will overload with shaft thrust.
- Thermal growth of the shaft can cause the same problems as shaft thrusting.
Problems with the pump operation that causes high heat at the mechanical seal faces.
- Operating off of the best efficiency point (BEP) is a major cause of high heat and vibration in centrifugal pumps. The degree of the problem is determined by the L3/D4 ratio of the shaft.
- Operating too close to the product’s vapor point can cause cavitation problems.
- Running dry will cause lots of heat.
- Pumping a tank dry.
- Losing barrier fluid in a dual seal application.
- Vacuum applications.
- Vertical pumps not vented in the stuffing box above the lapped seal faces and dynamic elastomer.
- The liquid you are pumping is not a lubricant.
- Shutting off the flushing water.
- Pump out rings on the back of the impeller running too close to the pump back plate can cause a vacuum in the pump stuffing box.
Other causes of high heat in the stuffing box.
- The shaft or sleeve is rubbing a stationary component.
- The seal gland.
- The restrictive or thermal bushing in the bottom of the stuffing box.
- The bushing in the API (American Petroleum Institute) gland.
- A wear ring
- A protruding gasket.
- A fitting.
- The stationary portion of a mechanical seal.
- The shaft or sleeve is not straight.
- It is bending because the pump is operating off of its best efficiency point.
- It is bent. This sometimes happens when the sleeve is removed.
- The rotating assembly is not balanced.
- The shaft never was straight.
- There is not enough circulation around the seal.
- Install a large diameter stuffing box. You should be able to get at least 1″ (25 mm.) all around the rotating unit.
- Connect a recirculation line from the bottom of the stuffing box to the suction side of the pump. You can do this in almost every case except:
- When you are pumping a product at its vapor point
- If the solids have a specific gravity lower than the fluid
- If it is a Durco pump where the impeller adjusts to the back plate
- When you are using a double ended design where the stuffing boxes are at suction pressure.
- The stuffing box cooling jacket is clogged.
- There is no carbon restriction bushing in the bottom of the stuffing box and you are using the cooling jacket. The restriction bushing is needed to slow down the heat transfer to the stuffing box area.
- Loss of an environmental control.
- The flush is not constant. The pressure is changing.
- Quenching steam or water has been shut off during pump shut down.
- The dual seal barrier or buffer fluid is not circulating.
- The cooling jacket has become clogged by calcium in the hard water. Try condensate instead.
- The filter or separator is clogged.
- Either the suction or discharge recirculation line is clogged.
If you are using dual seals remember that two seals generate twice as much heat, and conventional cooling may not be sufficient. Contact the manufacturer for his recommendations when using convection tanks and dual seals. You may need a “built in” pumping ring or some type of forced circulation between the seals.