Seal Types

Mechanical Seals

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.

There are two basic cases to be made out for the use of rotary, fluid sealing technologies.

The Environmental Case

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.

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.

The Economic Case

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!

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. This I promise!

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. In the enlightened 1990's 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. Whilst working for one of these majors in Saudi Arabia I found that it was common to find whole refineries using one manufacturers' seal. Long way from home, gutsy job, $millions at stake in oil revenues every day, it made a lot of sense for the engineers concerned. Only one company to deal with, lucky for some of them it was mine and my expertise was part of the deal! But there are many designs of seal and some I would think of as cheap and not so nice could give some of you 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.

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.