Why use a mechanical seal? After all is said and done, its easy to stuff a few extra bits of packing into a leaking stuffing box, and it doesn't require any skilled help to achieve this, does it? In this section we take a look at some of the reasons why you should be using seals. The Economic case and the Environmental case as well as considering some of the seal types available for general use.
It takes a lot of skill to pack a pump properly with soft packing.
There are two basic cases to be made out for the use of rotary, fluid sealing technologies.
We all have a responsibility to conserve and protect. Conserve scarce commodities and to protect the environment from pollution. A major spill is news because it is dramatic but every day, millions of glands leak chemicals into the environment. You can stop those leaks and avoid cleanup costs.
Have you thought about what that gland packing is doing to the shaft of your pump? It works as a brake, gripping the shaft and causing more power to be absorbed in the unit. The extra power consumption of the gland contributes to the "green-house" gases effect because more power has to be generated at the power plant to drive your pump.
As an experienced engineer you will know that the overall thermal efficiency of the power plant is much less than 35%, so if you can reduce your take-off by reducing the demand by replacing your glands with seals, less fuel is going to be consumed. Less fuel less emissions and less overall cost of running your plant. There is lots more to think about but space is limited so let's get on.
If you are not convinced that environmental pollution is your problem, loss of hard cash from your pocket or that of your company should be!
Water is becoming a scarce commodity. Let me re-phrase that - clean water is becoming a scarce commodity. For example, boiler feed water has to be at a high standard of cleanliness and chemicals are added to it to ensure that the water quality remains high. Make-up water is usually cooler than the water circulating in the system so additional fuel is required to heat the makeup water. It all costs money. Water is paid for on amount used. More cost! But you'll look at the pump in the corner dripping away and think that doesn't seem too bad. Ever done the mug test? A coffee mug holds 300cc. It is a simple matter to collect the leakage and note the time it takes to fill one mug.
One drip/second = 5,256 Litres / year
3 mm stream = 315,360 Litres / year
6 mm stream = 630,720 Litres / year
One drip a second is the standard rate for a properly adjusted packed gland : it leaks water, chemicals, and heat. Leaks usually get worse so look at the chart and now tell me if a leaking gland is inconsequential! Let's do another sum - how many leaking glands are there in your plant? Not all packed glands hold back water ... there may be more costly fluids leaking away. Each leaking gland is contributing to hard cash overhead expense. Packing is cheap, to buy, to fit, but its running cost is hidden and can be very expensive.
A mechanical seal appears expensive to buy when compared with a packing ring, but properly installed a seal will run for many years. The optimum life of a seal is the period between major overhauls of the pump unit. A seal that fails early by this criteria is in need of investigation. The criteria for a failed seal is one in which the running faces are not worn down to their designed minimum. However, an engineer does not want to spend money on a super seal that will last virtually forever because that will also not prove to be cost effective. When a seal fails it is possible, with experience, or the aid of this web site, to determine the cause of failure and to rectify that fault.
I was asked to select a seal for a water pump working in a quarry. The engineer had been plagued with seal failures for many years on this pump. His success criteria was that the seal should run from tear down to tear down (12 months). I selected a seal which was ten times more expensive than the one he had been using. It was fitted over the Easter Holiday 1982. Over a year later actually the week after the Easter holiday 1983 he rang me to say that the seal had failed. I reminded him of my promise that the seal would run for 12 months trouble free. He calmed down and started remembering, I told him that actually he had gotten an extra week over my promised 12 months! The increased price of the seal was around $400 but the saving in cost through not having to replace the seal several times in a year was over $1,500. The whole plant soon became converted to seals because it is possible to show a cost benefit analysis for every application.
It is often the thought that seals are expensive that prevents the engineer from opting for them. The same applies to pump manufacturers. Ever wondered why your plant is fitted with a particular pump make, each with a packing gland? In a word, competition. Whole life costing is becoming the way to assess a particular project's initial cost, but in the real everyday world engineers are facing the consequences of short sighted least cost solutions to immediate problems. But now you do not have to continue living with these problems if you look at the situation of your plant leakage in a business-like manner.
This web site does not represent any one company manufacturing seals. There are good logistical and economic arguments for standardizing on one manufacturer so long as they are major enough to run through all the applications you are likely to need. There are many designs of seal and some I would think of as cheap and not so nice could give excellent service. So this is not about price, but very much concerns cost. Balancing the cost of the seal installation against the outcome compared with the alternative. There is a wide range of materials to choose from. The range encompasses small variations in generic materials such as carbon, or o-rings and different metals used to cope with the conditions that faces the seal. I am not encouraging you to experiment blindly but to think the problem through and choose your materials carefully.
We are not going to look at the materials in detail here. For that information pop over to seal troubleshooting
I have not listed all seal types, the contact-less gas seals for instance are not covered here, this is because they fall outside the general seal types I aim to cover. For details on highly specialized seals of this and other types contact your favored manufacturer for details. In the links section of this site you will find hyper links to some manufacturers.
Now go look at the various seal types that are available to you for general use. In these sections you will find explanations of seal types and some of the problems associated with them.
Mechanical Seal Types