Why Use Mechanical Seals??
A question that comes up often is, “why should I use mechanical seals instead of packing when packing is cheaper?” There are some advantages that are obvious and easy to understand, and several others where the benefits may not be so readily apparent, so let us explain.
Cost of Product
With the right kind of materials of construction, an ‘acceptable’ leakage rate of as low as 1 drop per minute of sealed product per inch/25mm of outside diameter of shaft can be achieved. These numbers are based on an ideal scenario but, as experience tells us, the leakage rates can be considerably higher than this. It is worth considering that a gland leakage rate of just 1 drop per second is a loss of around 128 litres / 32 US gal per month or 6.6k litres / 1.6k US gal per year under ideal conditions. The truth is that most packed pumps leak significantly more than this, typically 9,500 litres / 2,375 US gal per month or 114k litres / 28k US gal per year for every pump!
Correctly designed, specified, and fitted mechanical seals have no visible leakage and therefore no product loss. If a manufacturers gross profit margin is low, or if a production cost is high, eliminating process leakage can have a significant impact on plant profitability.
Mechanical seals reduce the cost of lost product.
Disposal of Effluent and Environmental Issues
Dependent upon the type of product, the waste disposal costs can be significantly higher than the initial cost of manufacture. All we need to do is look at our own domestic water bills to substantiate this. Sewage charges are significantly higher that the initial cost of the product and this is only water.
With industrial waste product the options are either:
1.???????? Send the waste product to recovery, clean it up, send it back to process.
2.???????? Send the waste product to effluent treatment, clean it up, then throw it away.
As soon as any liquid goes to recovery or effluent treatment, the costs are significant, irrespective of its initial costs.
There is often an acceptance in manufacturing that “it’s only a bit of water” and that reducing water usage is not of significant importance. In a recent investigation at a food manufacturing plant, the water usage was found to be approximately 1m USG per day. The cost of purchasing the water was $1.1m per year and the cost of treating the resultant effluent $1.2m per year.
Minimising liquid to effluent treatment or recovery, directly reduces and can have a significant long-term effect in improving plant operating costs.
From an environmental perspective, when you consider that approximately 75% of the surface of the planet that we exist on is water, it is frightening to realise that only 3.5% of that water is freshwater. Unfortunately, most of that water is locked in polar ice and glaciers, leaving less that 1% available to all the life on the planet. With a growing demand for water from a growing population, there is no such thing a “only a bit of water” anymore.
Years of continuous leakage of process from packed glands can contaminate the concrete floor of the process area, which in turn has the potential to contaminate the subsoil, and ultimately the water table, giving rise to secondary pollution.
Companies are under extreme pressure and constant observation on environmental issues. Governments are continuously under national and global scrutiny to ensure that manufacturers accept the responsibility for the effects their products and processes have on the environment. Businesses themselves often also have environmental targets to meet. Be warned, legislation is here and environmental standards such as ISO14001 will become a requirement for business as environmental concerns for the planet grow.
As mechanical seals have no visible leakage, they therefore have minimal effluent disposal costs but have substantial effects on reducing and eliminating plant emissions and secondary pollution concerns.
Power Consumption
Gland packing can consume as much as 6 times more power than a balanced mechanical seal. Running rotating equipment with packing in the stuffing box is like driving your car with the hand brake on. Would this affect your fuel consumption? Of course, it would, and so packed glands therefore greatly increase power costs. In some cases, pumps can be made to run at a lower speed and efficiency, increasing costs still further.
Mechanical seals reduce power consumption when compared to gland packing.
Bearing Life
The single major cause of bearing failure in rotating equipment is contamination of the lubrication. Up to 50% of bearing failures are attributed to either moisture or particulate contamination.
As packing is designed to leak and is in close proximity with the bearings, they are subjected to constant stream of leakage and source of contamination. Any reduction in contamination will greatly increase the Mean Time Between Failure (MTBF), reducing maintenance costs.
Bearing contamination can come from many sources, but two “old favourites” are:
1.???????? Product leakage.
2.???????? Cleaning down the equipment in the interests of good housekeeping.
In a study conducted by Mobil Oil it was discovered that water contamination of 0.002% reduces the rated bearing life by almost 50%.?This is the equivalent of just 2 drops of water in a litre or quart of oil.
No leakage equates to longer bearing life and less downtime costs. Many mechanical seal failures are attributed to excessive shaft movement caused by failed bearings.
As an example, a pulp and paper manufacturer were having regular reliability issues with their Hard Wood Pulper that was gland packed. The regular leakage was causing maintenance issues and seeing large amounts of product being lost onto the factory floor.
?The pulper was originally fitted with aramid packing and was being re-packed every 6 months. With the pulper being vertically mounted, all leakage from the packing washed out any grease in the bearings leading to new bearings and new shafts being fitted every time the shaft was re-packed. The leakage was also causing large amounts of stock to be lost and was making the drive belts slip increasing power usage to operate the machine.?
Replacing the gland packing with a single mechanical seal increased the MTBF from 6 months to over 6 years and helped save over ￡137,000 in this period, without including the cost of lost product.
Mechanical seals improve the life expectancy of bearings.
Shaft and Sleeve Wear
Most rotating equipment is fitted with repairable wear sleeves in the region of the gland packing to acknowledge the fact that packing is designed to wear the shaft in this area. If a replaceable sleeve were not fitted, the shaft would need to be replaced or rebuilt in the worn area. This would be both expensive on terms of spares holding or extended downtime whilst the shaft is repaired.
Broken shafts are not uncommon and are generally caused by fatigue due to the constant bending of the shaft under operation.
From personal experience I can attest that shaft sleeves can also be notoriously difficult to remove. This can be extremely time consuming and may even lead to damage of the shaft itself.
Dependent upon their size, complexity of design and materials of construction, replacement sleeves can be expensive to manufacture, often costing hundreds of pounds, euros, or dollars to purchase.
Regardless of whether a sleeve is used or not, having to hold a stock of either shafts or sleeves can be expensive along with the downtime whilst shafts and sleeves are repaired.?
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Correctly designed mechanical seals do not wear expensive shafts or sleeves.
Product Dilution and Associated Energy Costs
Gland packing often requires the supply of a clean flushing medium to the lantern ring. This was originally done to prevent the ingress of air into a system under vacuum and was also found to cool the packing as it generated friction against the shaft. However, the use of an external flush will result in some form of product dilution. This dilution may be acceptable to the process but if not, additional costs may be incurred for its removal at a later stage. If you pump a liquid but manufacture a solid (as in the paper industry) there is an inevitable cost associated with extraction and the possibility of additional costs for effluent and recovery.
Choosing the right mechanical seal can eliminate product dilution resulting in a better quality, purer product with no additional removal costs.
There is also the possibility of re-heat costs due to the addition of large quantities of external flush water. The energy requirements to keep process at the required temperature as additional external fluid is added, can run into hundreds of thousands of additional and unnecessary kilowatts per year, per pump or millions across a site. As an example, to raise the temperature of added flush water in one pump, with a flow rate of 8 Lt/min, to a process with a temperature rise of just 40oC / 70oF, requires 195,000 kWh or 665,660,000 btu per pump per year to do so.
That’s a lot of kWh or BTU, in fact it’s enough energy to heat the Empire State Building for 10 days from just one pump!
If this energy is being generated through a gas-boiler with 85% efficiency, this corresponds to additional emissions of 42 Tonnes CO2e per pump per year, equivalent to 10 cars.
It is not unusual to see boiler feed and condensate pumps that are packed. The costs associated with these applications are huge.
Firstly, the fluid in process is expensive to produce, it is de-mineralised water.
Next, the water must be treated to raise its pH to around 12 or 13 to prevent corrosion of the boiler tubes.
Lastly, the ambient temperature of the water has to be raised in order to achieve the required operating temperature, which requires billions of Joules or BTUs of energy to do so.
Because of all these associated costs, leakage of the water needs to be kept to a minimum.
So, what happens if the boiler feed or condensate pumps are packed?
The pumps now leak water because they are packed, and every litre or gallon that leaks out must be replaced, and its de-mineralised water so that’s expensive to do.
The replacement water is now considerably colder than the water in the boiler system, so now energy is required for re-heat, to bring the water up to operating temperature. It requires 4,200 Joules to raise the temperature of 1kg water by 1oC or 1 BTU to raise the temperature of 1lb of water by 1oF.
If we were to have a boiler system operating at 120oC, to raise the temperature of just 1kg of replacement water from 10oC to 120oC would require 459,800 Joules to do so. With pump glands leaking thousands of litres per year, the energy re-heat costs are significant. With power station systems operating at 300oC, the energy costs are astronomical.
Choosing the correct mechanical seal will substantially reduce re-heat costs.
Maintenance Costs
To ensure that packing operates as efficiently as it should it requires constant attention. Too loose and leakage rates are unacceptable, too tight and the packing can burn out on start-up, increasing leakage, wear on the shaft or sleeve and increasing power consumption still further.
Packing is not simply just greasy rope but is treated as such. It is often treated as a low-skill requirement product too, which it is not. Maintenance must be constant and conscientious throughout the life of the packing otherwise leakage will be excessive and life expectancy short.
However, packing still needs to leak and the resultant leakage produces secondary maintenance costs:
By eliminating leakage, secondary maintenance is minimized by reducing corrosion, cleaning, and painting. By using a mechanical seal, these secondary maintenance costs are also greatly reduced.
Other than keeping seal support systems filled on dual seals, mechanical seals need no routine maintenance.
Housekeeping.
If there were no leakage there would be reduced corrosion, reduced cleaning and reduced painting required.
If you think housekeeping is not an issue, try this one! Go home and take the mechanical seals out of the washing machine and dishwasher pumps in the kitchen. Then explain to everyone in the house that one of you will have to constantly mop the floor because they leak, take the back off every week to adjust the packing to reduce the leakage, and occasionally remove it all, replace it and start all over again. I hope you have a good supply of mops and a fabulous relationship with everyone you live with and that you don’t live in an apartment that is not on the ground floor too!
Secondary maintenance costs would be greatly reduced.
Safety.
Packing leaks, as it is designed to, and it leaks the “sealed” product to atmosphere. As we have already agreed this is becoming more and more unacceptable especially if the product is corrosive, toxic, explosive etc. We are all aware of the effect fugitive emissions are having on our planet, and the resultant “Global Warming”.
All products classified by fugitive emission or hazardous should be double sealed.
?By using mechanical seals, airborne pollution is greatly reduced; double seals can achieve zero product emissions.
If your plant is using packing to seal pumps, then there are so many opportunities to save both time and money.
As people we like to see real world examples, we have put together a video and a pack of some of our most recent case studies. Give a watch - www.sealselection.com/packing-vs-seals
Authored by Chris Dean
Head of Marketing and Executive Assistant at AESSEAL Plc
1 年Brilliant article Chris Dean! ????
C-Suite & Board Member Conduit | Senior Executive Liaison
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