Have You Selected Wrong Materials for Chemicals?

by : Thomas Yoon

Chemicals are very much a part of our lifestyles. Every household detergent, solvent, and bleach that you use in your homes is a result of some production efforts from manufacturing plants somewhere in the world around you. Fertilizer, automobile radiator coolant, shampoo, soap, insecticide, paint solvent, lubricants, fuel oil are just a few that I can name right now. I’m sure you can find more around you, but you get the point. We use chemicals everywhere.

Anyone who has visited a chemical processing plant is sure to notice the many pumps, agitators, tanks, piping, and valves that are installed there. Liquid have to be transferred from one place to another. Pumps are therefore very important in a chemical processing plant. Without them the chemical processing plant will literarily come to a halt!

One of the main jobs for Plant Engineers is to maintain the numerous pumps installed at their plant. These pumps can count into the hundreds or even thousands, depending on the size of the plant. So you should realize that to do a proper preventive maintenance, it is no mean task. There must be regular schedules, proper tracking of jobs done, available manpower and skills, suitable tools, replacement parts, materials and a proper system of administration of all these.

Sometimes, even with all the maintenance activities being carried out, pumps do fail. And when they do, plant engineers will have to find out what causes them to fail. Especially with new pumps where there is very little record trend of breakdown, engineers will be hard pressed into finding solutions for this. This is when experience helps in pinpointing the cause(s) of the failure.

Pumps or other machinery will give tell-tale signs when they are not working properly. An observant pump user will be able to avoid major breakdowns or damage if the problem is corrected early.

In order to solve any pump problem, we need to notice the symptoms carefully so as to determine the most likely causes. Instruments like pressure gauges are very helpful and should be installed in the pumping system.

Very often we do have to rely on our 5 senses to determine the exact symptom. Normally, pump problems can be classified into:

1) Suction Related,
2) System Related, or
3) Mechanical Related.

It can also be a combination of these.

Most of the system related problems occur because of design flaw. For example, the designer may have chosen the wrong pump whose characteristic does not match the system requirement. Suction related problems are usually caused by air locks that are due to a variety of reasons. I will not mention them here.

The effects of mechanical related problems could manifest themselves as suction related problems - air leaks in the system, worn out impellers, and mouth rings - but the most common symptom is the presence of vibration and abnormal noise in the equipment.

However, there is one area where we seldom focus on - the effect of chemicals. Was the material selected able to withstand the chemicals? If you have done all your designing right and you still find that leaks are occurring so frequently, chances are that the materials are failing due to chemical reaction. Signs of corrosion at the seals are a strong indication of material failure due to wrong usage.

Nowadays, waste treatment plants or even process plants are called upon to process strong acids, alkalis, oxidizers, solvents, waste, slurry and other corrosive and abrasive chemicals. And it is not only pumps that are affected. Agitators, storage tanks, and piping need to be compatible to these chemicals. If the wrong material is selected, it can lead to dangerous and widespread consequences of chemical spills, emergency evacuation, pollution, environmental damage and other disruptions to the production facility.

Engineers in such chemical processing plants need to know what materials are suitable to be used for their process. It is much more complex than just selecting materials for water pumps. Much detailed and careful selection choices based on the chemicals, the temperatures (because some of the plastic materials can weaken at temperatures that are considered normal for metals), chemical reactions, safety, spills and many others have to be taken.

The aim of any maintenance personnel is to lengthen the lifespan of equipment under his care. Selection of chemical resistant materials is not a one-cure-fits-all solution. Some materials are not affected by certain chemicals but can be damaged by others. Some have good mechanical strength while others cannot last long without reinforcement. Usually, materials that can withstand many types of chemicals are very expensive.

Cost considerations need to be taken into account when choosing a material for a certain application. If there is a choice of materials for a certain application, it makes better sense to choose cheaper materials if they can perform just as well.

There are several components to check for when selecting suitable materials:

1) Elastomers for flexible parts like seal rings and gaskets
2) Metal parts like shafting, springs, holders, bolts & nuts, and pressure gauges
3) Plastic parts like housing, piping, impellers, and covers

Metals have good mechanical strength that can last very long in operation. In fact some parts need to be made of metal, e.g. bearing housing and shafts. Pump housing made of metal can be casted and machined easily. They are able to withstand abrasive fluids and rough handling without any other reinforcement. One very important characteristic of metals is heat conduction. If the chemical to be pumped is hot, metal is always the choice.

Plastics have become the better choice for many corrosive chemical applications because it is more resistant to chemical action than metals. When using plastics to replace metals, you have to compromise on the mechanical strength of the pump. If the pump does not encounter much rough handling or abrasion, plastics can be used. Sometimes plastics are used to coat metal parts. These are fine as long as the plastic coating remains intact.

Elastomers are the flexible materials that are needed for sealing the pump parts. There should not be any compromise here. Laboratory tests on the suitability of elastomers with certain chemicals should be followed. Unsuitable material used can cause leaks that can lead to other failures in other parts.

With so many chemicals in use today, how do we know what materials can be used for what chemicals? Sometimes liquids to be pumped contain chemicals that are both corrosive and abrasive. Should we choose a plastic or a metal housing? Sometimes chemicals may become hot either through the process or through mis-operation of the system - perhaps, somebody forgot to open a valve. Plastic parts can weaken at high temperatures.

It's only through the result of test and actual operation that we are able to know what materials to use. It is good to know what to expect when we make a decision for a choice of materials to use. A good choice may involve some compromise.

To help you make a good decision, check out http://www.free-engineering.com/searchdata.htm for a list of chemical resistance of materials.