Why packed pumps should be converted to a mechanical seal. The advantages of using a split design
Packing conversion is a subject that’s become more important in recent years. In the following paragraphs I’ll 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.
- The reasons you should convert to split mechanical seals, rather than the type that requires taking the equipment apart .
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 off moisture laden air re-enters through this vent.
At least three costs are associated with packing sleeves.
- To install them the shaft outside diameter often has to be reduced. Reducing the diameter will weaken the shaft
- Packing damages the shaft 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 as they 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. This is an easy test to 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.
- In the event you do not record a drop in amperage you’ll 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’ll 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’s produced during the cutting and fitting process. Unused pieces are often thrown into the trash rather than returned to the stock room.
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 water table. Leakage to the air contributes to airborne pollution and all of the problems associated with contaminating the atmosphere. Fugitive emission laws are restricting 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’s 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 this air ingestion will lower the PH of the boiler feed water causing the addition of more chemicals and additional boiler blow downs. It will also add additional cost because de-aeration 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 double sealed. Packing these pumps should be classified as a criminal activity.
Seals are self adjusting
Once the mechanical seal is installed correctly there is nothing to do unless the impeller needs adjusting. Packing, unless it’s live loaded, needs frequent adjustment to prevent excessive leakage.
We are all familiar with the utility man that has a roll of electricians 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.
Now that you have made the decision to convert to mechanical seals you have a couple of more decisions to make.
- What brand of seal should you select?
- What seal materials should you choose?
- Will you need an environmental control?
Split seals are the one item that seal users said they always wanted. No one wants to take apart a piece of equipment if it’s not necessary. Split seals are available from all of the major seal companies, but not all seal materials and sizes are available from each one. Modern designs are priced comparable to a typical single cartridge seal with spare parts priced lower than most single, non cartridge seals.
A split mechanical seal is defined as having all components split at the time of installation. It’s important to note that no dynamic elastomers should be glued together because the hard spot that develops will not allow the elastomer to be functional.
Split seals can be used to convert packed pumps to mechanical seals and to replace solid type mechanical seals. These split seals are available in both a rotary and stationary version. If you get the choice select the stationary type.
We’ve already discussed the reasons you’d want to convert a packed pump to a mechanical seal; now we will look at a few areas where the split seal has a real advantage over solid seals.
Some fire codes will not allow you to remove the packing from a fire pump. The split seal can easily be installed outside of the stuffing box with the packing left inside. Fire pumps leak a great deal and it’s not unusual to have a “jockey pump” run twenty four hours a day trying to keep the fire main pressurized as this leakage occurs. Many fire pumps have been converted to a mechanical seal and in just about every case the “jockey” pump has stopped running.
Sometimes it’s just too difficult to pull a pump to change the packing sleeve or mechanical seal. Most split seal applications can be done in less than an hour, with the pump left in place.
Any time down time is expensive.
Whenever a solid seal wears out or fails, it takes a considerable amount of time to pull the pump and change the seal. Split seals do not have this problem. In some cases this “down time” can cost thousands of dollars.
The worker can fix the leak and then get out of the area in a hurry. Radioactive environments are an example of this problem. In many cases the old packing and sleeve, or failed mechanical seal does not have to be removed to install a Split seal.
The coupling and motor do not have to be pulled and therefore realignment is no longer necessary. Even if you have the latest laser equipment, it still takes a long time to align a driver and pump properly.
No need to overhaul the pump most of the time.
Good split seals will not wear a shaft so there is no need to pull the bearings unless they have been damaged. You change only the seal, not the shaft and bearings. To prevent damage to the shaft you should replace the bearing lip or grease seals with positive face seals or the labyrinth type that work better and do not damage expensive shafts.
No damage to the pump during seal replacement.
Since the pump is not being disassembled there is less chance of damaging something during the seal or sleeve change. Often there are no spare parts available for some of the older pumps. Usually there is nothing wrong with the pump&emdash; only the leakage is the problem.
The pump cannot be repacked. The stuffing box or sleeve is too far worn.
In many cases the inside of the packing chamber has corroded or the metal has fatigued causing the packing to rotate with the shaft. The face of the stuffing box can easily be repaired with available commercial products and a split seal installed outside of the packing chamber. There is seldom any need to remove the damaged sleeve during these installation.
Mixers and agitators.
Disassembly of this equipment is always a problem. Many times it pays to install a split bushing in the bottom of the packing chamber to help stabilize the shaft. For those mixer designs that do not require emptying to change the seal a split sleeve can be installed under the split seal.
Systems that have to be sterilized
In some instances a split seal can be sterilized and then installed without having to sterilize the entire system. This often happens when the pump has to be removed from the piping.
The insulation does not have to be removed.
Many pumps have been insulated with asbestos packing and its removal is an involved and costly process. Split seals can usually be installed with no need to remove this installation.
Seal repair and disposal problems.
Split seal components are easily replaceable and do not take up any volume. There is no need to send seals back to the manufacture or any other facility for rebuilding. Repair can be done by the mechanic at a cost that is usually lower than comparable size solid seals. Recent “Right to know laws” have created an urgent need for these designs.
No multiple trades needed
Most split seals can be installed by one man in less than an hour. No need for multiple trades and the multiple work orders involved.
Split seals can back up existing seals.
If a dual seal is necessary, you can usually install a split mechanical seal behind the present seal to protect the product and area in the event the present seal fails. A convection tank using anti freeze or any compatible liquid can be installed between the seals as a barrier fluid. This system works well with exotic metal pumps also. A 316 stainless steel split seal can often back up an exotic metal seal because it will only be activated when the first seal fails.
You can install a split seal on a leaking pump and stop the leakage until you have time to replace the seal that was specified for the pump. If there is not enough room between the seal gland and the first obstruction you can cut off the existing gland and leave the leaking rotary unit attached to the shaft inside the stuffing box.
Using split seals you can convert packed pumps at a much faster rate. In fact there’s no longer any need to wait until turn around time to fix or convert leaking pieces of rotating equipment.
There’s no longer any need to speculate if a seal will work in a given application. You can always install a split seal and learn the answer during the current work shift. A typical split seal application is accomplished in less than one hour with thirty minutes being more common as the mechanic gains experience.
- On February 17, 2018