CONVERTING PACKED PUMPS TO A MECHANICAL SEAL, THE ADVANTAGES C058
Packing conversion is a subject that has become increasingly more important in recent years. In the following paragraphs I will try to divide the subject into three areas.
- The “obvious” dollar savings that can be realized by making the conversion.
- The “non-obvious” additional reasons for converting packing to a mechanical seal.
- The reasons you should convert to split mechanical seals whenever possible, rather than the solid type that requires taking the equipment apart .
We will begin with the savings that are real and easily measurable.
Cost of the product.
- A leak equivalent to the smallest steady stream that you can produce, is equal to one to two gallons (3,5 to 8,0 liters) per hour. At a cost of $0.25 per gallon this would come to $3285.00 each year. Enough money to purchase several mechanical seals.
Treatment of waste.
- Depending on the type of waste, the cost of disposal can be several times the cost of the product that was leaking. Household sewage is a good example. The sewage portion of your water bill is normally larger than the water portion.
- Chill water, hot water, cooling towers and boiler feed water are examples of systems that need chemical addition to protect the equipment against corrosion, bacteria etc. These chemicals are very expensive and add to the cost of the leakage.
- The major cause of bearing failure is contamination of the bearing oil. As little as 0.002% water in bearing oil can reduce the rated bearing life as much as 48%. Most of this water comes from packing leakage and the water hose you use to wash the leakage down the drain. When the pump is running, heated air vents out through the oil filling connection. At shut down moisture laden air re-enters through this vent.
- To install a sleeve the shaft diameter often has to be reduced and in the process of doing so the shaft is weakened.
- Packing damages the sleeve as it removes the corrosion resistant metal’s protective oxide layer.
- Because sleeves are so hard to remove we generally end up replacing the bearings at the same time because the bearings are often destroyed during the sleeve removal process.
- Packing a pump is like driving your car with the emergency brake engaged. Although the car would run, it would consume more gasoline. On the average packing consumes six times the power of a mechanical seal. Here is an easy test you can perform :
- Pack the pump properly and run it long enough to stabilize the operating temperature and pressure. Take an amperage reading at the motor or starter (not the breaker) when the pump has stabilized. You should also record the pump rpm. at this time.
- Remove the packing, install a split mechanical seal and record the amperage difference. Combine this data with the amount of money you have to pay for electricity and the results will be obvious. Be sure to use a split seal so hat you will not alter any pump internal clearances.
- In the event you do not record a drop in amperage you will notice an increase in motor speed. Many marginal motors are being “bogged down” by the friction from five or six rings of packing.
Eliminate the flushing fluid.
- Depending upon the flushing fluid you were using, the cost can be very high and often unreliable. If you will install an oversized seal chamber and then connect a line from the bottom of the stuffing box back to the suction side of the pump most flushing fluids can be eliminated.
- Caution : Do not install this line if you are pumping close to the vapor pressure of the liquid, as the lower pressure may cause the fluid to vaporize in the stuffing box or between the lapped seal faces.
- In the event a flushing fluid is required, only a small amount will be needed with a balanced O-ring type mechanical seal. One to two gallons (3,5 to 8 liters) per hour (not per minute) would be typical if the springs are not in the fluid and the stuffing box internal diameter has been increased.
Stop product dilution.
- As mentioned in the above paragraph, if you have eliminated flushing there will be no product dilution. Once you dilute a product there is additional cost involved in removing the diluent. This is normally done through an evaporation process that involves costly power and/or the creation of a vacuum.
- If nothing leaked there would be very little corrosion and repainting would never be necessary. Most leakage comes from valves, flanges and rotating shafts. They can all be sealed to prevent leakage.
Packing material waste.
- Find out how much you are paying for these modern packings and then observe the waste that is produced during the cutting and fitting process.
- Multicraft, operator maintenance, and contract labor are becoming a way of life in many plants. The life of packing is directly related to the skill of the man that packed the pump, and the skilled craftsman is disappearing fast.
Additional reasons to convert from packing to a leak proof mechanical seal.
- Leakage to the ground ends up in the ground water table. Leakage to the air contributes to airborne pollution and all of the problems associated with polluting the atmosphere. Fugitive emission laws are restricting some types of leakage to parts per million.
Vertical pump applications
- Vapors escaping from the packing gland of a vertical pump are the major cause of electrical motor failure. The vapors not only contaminate the grease lubricant, but are the main cause of damage to the insulation of the motor’s windings.
- There is no packing that can seal vacuum. Flushing water looks like it is doing the job but testing has shown that flush water can go down one side of the packing as the vacuum lets air come in the other side. In a condensate pump carbon dioxide (CO2)ingestion will lower the pH of the boiler feed water causing the addition of more chemicals and additional boiler blow downs. Air ingestion will also add additional cost, because deaeration is almost always necessary to lower the oxygen content of the water.
- This is the reason you use a mechanical seal on the water pump of your car and the pump in your household washing machine. Changing sleeves and packing in these applications would not only be too costly but the leakage would be intolerable.
- The list of hazardous materials is getting bigger every day. Unfortunately the hazard is not always visible. Human beings should be breathing nothing but clean fresh air. Keep the other stuff inside the machinery where it belongs.
- Any product classified as a fugitive emission or hazardous should be dual sealed to protect the environment and any personnel that might be close by. Sewage, with hydrogen sulfide and methane gas always present, is a good example of a product that should always be dual sealed. Packing these pumps should be classified as a criminal activity.
- Recent legislation is restricting the leakage of some fluids to as little as a few hundred parts per million. Packing is out of the question in these applications and dual mechanical seals are the only sensible solution. If the product is a clean lubricating liquid, a magnetic drive or canned pump is another logical choice.
Seals are self-adjusting
- Once the mechanical seal is installed correctly there is nothing to do unless the impeller needs adjusting. Packing, unless it is live loaded, needs frequent adjustment to prevent excessive leakage.
We are all familiar with the utility man that has a roll of electrician’s tape in one pocket and a twenty six inch adjustable wrench in the other. He is the one that adjusts the packing on the back shift and weekends.
Converting to mechanical seals is an excellent way to keep him away from your equipment.