Steam Traps - Care and Maintenance

Steam Traps - Care and Maintenance 

Steam traps have play a very important role in steam distribution systems. The service performed by steam traps is primarily to discharge condensate. Normally a steam trap can be easily and quickly selected by considering only the average operating conditions. However, an exact analysis of these conditions will give the proper data necessary for selecting the type and size for greater savings and proper plant operation. After the careful selection of the steam trap, it must be properly installed, tested, periodically inspected, cleaned and maintained to keep it operating efficiently.

Traps need cleaning periodically. A simple way to prevent dirt from entering is to drop a short length of pipe vertically below the supply to the trap (called a dirt leg) which can be cleaned easily and frequently.

Traps can be seriously damaged by scale or pipe comings in lines. A good practice is to install strainers ahead of the traps which should be inspected and cleaned frequently.

Traps are subject to severe wear if steam blows through continuously. They should be inspected for worn valve parts or a change in operating conditions.

When a steam trap fails to discharge, inspect the heating system and be certain that all units are drained with separate traps, thus guarding against short circuiting, loss of energy, and reduction of operating efficiency.

Traps operating under high pressure or superheated steam are often insulated in a manner similar to adjacent pipe lines. In such instances, they shall be fitted with dirt pockets, test valves, and drains.

Steam traps installed in areas exposed to climatic conditions will lose heat if not insulated and may freeze unless adequately protected. Discharge lines should be short and self draining and traps should be fitted with a drain tapping and valves.

Steam traps handling large volumes of air require more frequent inspection and proper venting for efficient operation. Vents shall be used to avoid air binding and ensure positive drainage. Gauge glasses shall be kept in proper repair, for they indicate whether or not the trap is working. Periodic cleaning and gauge glass replacement shall be considered as a high priority in the maintenance of steam traps.

All steam traps require protection from corrosion to prevent unnecessary deterioration. All valves, joints, and gaskets should be kept tight to avoid steam leakage and ultimate energy losses. For continuous and efficient operation. steam traps require periodic inspection and maintenance for purposes of eliminating foreign matter and obstructions in supply and discharge lines. Each steam trap at an assigned work station should be inspected as specified by the preventive maintenance program.

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Comparison between Gate valve and Globe Valve

Comparison between Gate valve and Globe Valve

1. Gate valves are used for On-Off control whereas Globe valves in addition can also be used for the flow regulation.
2. Gate valves offer very little resistance to fluid flow in fully open position and also have small pressure drop across the valve. Globe valves on the other hand have a high pressure drop even in fully open conditions and offer substantial resistance to fluid flow.
3. Gate valves of the same size are cheaper than globe valves.
4. Under very low pressure, i.e. 5 psi, light seepage would not be considered abnormal with a gate valve. On the other hand, Globe Valve will not leak under low pressure when they are shut off.
5. Gate valves because of their design, have very little fluid trapping in the line, but the globe valves have a larger amount due to the direction of flow.
6. Gate valves are unidirectional and can be put around in any way. The globe valves are not.

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Why we pass cold fluid to shell side and hot fluid to tube side?

If we pass hot fluid through shell then we will have a chance of heating loss to the surrounding.


However, practically this statement is not true.

The choice of shell and tube-side fluids for an heat exchanger are often governed by more demanding requirements which have implications on the safety, cost, maintenance time and feasibility of the heat exchanger and cannot be generalized as cold fluid on shell side and hot on tube-side. 

Some examples on how this choice is made might be - 

1-Frequent mechanical Cleaning - Tubeside to more fouling fluid, if mechanical cleaning is required. Note that chemical cleaning can always be done even on shell side. Mechanical cleaning on shells side however would require access to tube outer surface which in turn would require design features such bundle pullout possible / tube-tube gap/tube layout. It is more difficult than tubeside cleaning as it will require dismantling the whole exchanger. eg most Cooling water exchangers in process industry, which are frequently cleaned using hydrojetting.

2-Pressure - Tubeside to high pressure fluids (Shell side high pressure increases the thickness of shell and can have great implications on cost of the exchanger). Remember, shell cost forms the major cost of the exchanger in most cases.

3-Viscosity - Shell side to viscous fluids as turbulence, which can be more easily induced on shell side might be difficult (note viscous and fouling fluids are not essentially the same thing and are often confused as high viscosity implying high fouling). And then there are tube inserts which can give you that effect on tube-side as well.

4-Corrosive - Tubeside to corrosive fluid - Less components for it to see and eat up

5-Metallurgy - Expensive metallurgy on tubeside- Process requirements such as compatibility of your fluid or high or low temperatures might require you to use a particular metallurgy. It is always cheaper to use the more expensive metallurgy on the tubeside as it will require less of that metal.

This is not an exhaustive list and you can find more things in any chemical engineering text on heat exchangers for more info.

It gets tougher than this in real life where you might have competing requirements and this is often a choice between lesser of the two evils. To give you an example would be an  HF - Cooling water exchanger, where you will have to choose between a corrosive or fouling fluid to be kept on the shell side. 

To make things even more complicated, there might be cases such as some plants dismantle the exchangers and transport them to a different place dedicated for cleaning of exchangers for safety concerns. You therefore need much more information for this simple decision if you are designing an actual exchanger.

Source - Ankit Malhotra, Chemical Engineer, Experience in Heat exchanger design for Refining and Petrochem.

By another reference, it's not a good practice to always put cold fluid in the shell.

1- The most general practice is that you calculate the flow areas for both, shell and tube, and put the fluid with higher flow rate in the larger flow area. Otherwise, the pressure drop will shoot up, resulting in a poor design.

2- If any fluid is corrosive, you try and put it in the tube (This way, you only have to clean the inner surface of the tube. On the other hand, if it were in the shell, you'd have to clean the outer surface of tube as well as the inner surface of shell).

3- Generally hot fluids result in scale formation, and because of cleaning reasons explained above, they are put in the tubes.

So, it totally depends on the application, and as such there is no general rule.

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