Level 3 Heat exchangers and Pressure vessels Topic 1

Heat exchangers and pressure Vessels
What are heat exchangers and pressure vessels?
Heat exchangers and pressure vessels both hold liquid and/or gas under high pressure, although their working is somewhat different.
The heat exchanger transfers heat from one substance to another across a barrier that keeps the substances apart.
The pressure vessel holds water, gas or air under high pressure.
All heat exchangers are pressure vessels but not all pressure vessels are heat exchangers.
Examples of heat exchangers that are also pressure vessels are radiators in cars and pressurized water reactors in nuclear power stations.
An example of a pressure vessel is a water geyser in a home.
How a heat exchanger works.
The heat exchanger consist of an outer shell that contains a set of tubes and an inlet and outlet. The set of tubes can be a sealed unit or each tube can have it’s own inlet and outlet through which the liquid or gas flows. This liquid or gas is called the fluid and is usually water although air or other substances may be used.
Another substance, either liquid or vapor, flows around the outside of the tubes within the outer shell.
The water in the tubes is much hotter than the liquid or vapor around them on the outside. The heat is therefore transferred from the hotter water in the tubes e cooler liquid or vapor flowing on the outside of the tubes. In the process the water inside the tubes also cools slightly.
The inside of the tubes is called the tube-side while the outside of the tubes is called shell-side.

Heat exchangers use three types of flow layouts for the fluid or air flowing inside and outside the tubes.
• A parallel layout, in which both fluids flow in the same direction,
• A counter layout, in which the fluids flow from opposite directions
• A cross layout, in which the fluids flow perpendicular to each other.
Metal plates called baffles are positioned inside the shell of the heat exchanger to control the speed and direction of liquid flow around the shell. In order to make the flow of fluid longer, the tubes are divided into sections by partitioning plates in the header of the heat exchanger. These sections are called passes. The common pass arrangements are a one-shell, two tube pass and a two shell, two tube pass.
In a one-shell, two tube pass, the shell side fluid is designed to have a one-shell pass because it flows through the inlet on one side of the heat exchanger and then out through the outlet on the other side. It passes through the shell only once. The tube side fluid has its inlet and outlet on the same end of the exchanger. The fluid flow down one pass, up the head and back down the other pass, so it passes through the shell twice.
A two shell, two tube pass works exactly the same as described above, except that the shell side fluid also passes through the shell twice.
There are two main types of Heat exchangers
 Shell-and-tube heat exchangers
 Plate heat exchangers.
The Shell-and- tube heat exchangers:
This is the most common heat exchanger used in the industry. It consists of a bundle of tubes, called a tube assembly and it is contained in a cylindrical shell. These tubes are made of metal and the ends of the tubes fit into metal sheets called tube sheets. The tube sheets support the tubes and stops the two fluids from coming in contact with each other.
Shell-and-tube heat exchangers can be divided into two main groups
 Straight-tube heat exchangers
 U-tube heat exchangers
Straight tube heat exchangers has straight tubes and is generally used for heavy fluids that contain large particles. Because the tubes are straight it is easy to clean and replace them.
U-tube heat exchangers make use of straight tubes that are bend into a U-shape. As the tubes are bent it is not easy to clean or replace them. They are therefore used for fairly clean fluids.
The Plate heat exchangers:
The plate heat exchanger consists of a number of metal plates with small spaces between them. These plates are corrugated and are sealed or welded together to form a frame through which the two fluids can flow parallel in alternative channels.
Plate heat exchangers are very effective because the fluids flow over a large area.
You cannot use fluids that contain particles or deposit large amounts of scale because this will block the channels which will affect the flow of the fluids.
How a pressure vessel works.
A pressure vessel is usually a cylindrically-shaped or spherical container that holds liquids or gasses under high pressure.
The substance that a pressure vessel will hold determine the material the vessel is made of as well as its size, temperature level, shape and pressure level.
Pressure vessels used by industry are designed to work safely at a set temperature (also called design temperature) and pressure (also called design pressure)
Pressure vessels can be extremely dangerous if they are set to design pressure which they are not made to handle. If the pressure is too high the pressure vessel could explode. Pressure vessels is therefore a safety hazard and their use and operation is regulated in South Africa by the Occupational Health and Safety Act.
A pressure vessel has a maximum pressure rating at which it can operate safely. The vessel must have a pressure gauge and a safety valve. If the maximum pressure is reached the safety valve will open and release the excess pressure. Once the pressure is released the safety valve will close.
Why you need to maintain heat exchangers and pressure vessels.
• To improve the rate of heat transfer which is affected by dirt and scale build up.
• To make sure that they are safe to use according to the manufacturer’s specifications and the OHS Act
• To find and repair faulty or damaged parts before they cause serious injuries or death to operators or damage to plant or nearby buildings.
• To prevent untimely breakdown of the equipment.
Correct sequence of activities to follow.
Keep in mind that only a qualified person is allowed to carry out repairs on heat exchangers and pressure vessels.
It is also important to remember that a boiler or pressure vessel is normally the power source of a factory. The various other machines and processes that depend on the steam from the boiler room to function is very complex. To shut down the heat exchangers and pressure vessels in the factory for routine maintenance, permission can only be obtained from the resident engineer. Failing to obtain his permission can be very costly and even dangerous. Even emergency work can only be carried out with permission of the resident engineer.
You will use the following sequence of activities when you maintain pressure vessels and heat exchangers.
Step 1: Study the information and instructions on the Job card, reports and engineering drawings and examine the maintenance schedule.
Step 2: Inspect the heat exchanger/pressure vessel during operation to identify possible faults and parts to replace.
Step 3: Choose the necessary tools, equipment and safety equipment.
Step 4: Check the site and make sure it is a safe environment to work in.
Step 5: Isolate the vessel and make sure the system is safe to work on. Release pressure before commencing with the maintenance.
Step 6: Inspect the condition of the heat exchanger and pressure vessel
Step 7: Write a report on the results of your inspection.
Step 8: Test the heat exchanger and pressure vessel.
Step 9: Service the heat exchanger and pressure vessel.
Step 10: Clean and store tools and equipment.
Step 11: Write a report on the work you have done.
The implications of not following this sequence of activities.
The following problems may happen if you do not carry out the activities in the correct order.
 Removing the heads of the heat exchanger without shutting down that section of the plant could result in severe burns by steam and the blast from the furnace because of the pressure in the vessel. In addition to that the pressure in the vessel will turn the loosened parts of the vessel into missiles.
 You will seriously injure and burn yourself if you remove the tubes and bends of the heat exchanger without allowing time for them to cool down.
 You will waste time looking for the right tools halfway through the job if you have to stop working to go and fetch them.
 You can damage parts if you fit them in the wrong position.
FOLLOW THE CORRECT SAFETY PROCEDURES.
Safety in the worksite is especially important in areas that contain heat exchangers and pressure vessels. This equipment hold liquid, fluid and gasses under pressure. If you don’t maintain the correct pressure or depressurize the vessels before you work on it, you or someone else could be badly hurt or even killed.
The safe maximum work pressure of heat exchangers and pressure vessels.
All heat exchangers and pressure vessels have a safe maximum work pressure. This is the maximum pressure at which the vessel can still operate safely.
The maximum work pressure will be different for different types of heat exchangers and pressure vessels. The following factors determine the safe maximum pressure.
• The substance that the equipment will hold
• The material that it is made of
• The size of the vessel
• The shape of the vessel
• The temperature at which the vessel is operating
Heat exchangers and pressure vessels have pressure gauges and safety valves. When the maximum pressure is reached in the vessel, the safety valve will open to release some of the pressure. Once the pressure has dropped back to acceptable limits, the safety valve closes again.
The safety rules for working on heat exchangers and pressure vessels.
High pressure and the nature of the substances inside this equipment makes maintenance work on these equipment extremely hazardous. It is therefore important that you follow the applicable safety rules when you work with heat exchangers and pressure vessels.
The following rules should be obeyed at all times.
o Don’t loosen any nuts, bolts or fasteners until you have checked that all pressure in the vessel has been released. Also make sure that all the contents from the shell-side as well as the tube-side has been drained.. Failing to do this can result in serious injury or even death.
o Make sure the proper rigging or hoists is in place to lift heavy parts when you are working on big heat exchangers and pressure vessels.
o Cleaning solvents and fluid that you drain from the equipment must be handled with extreme care. Most of these fluids and cleaning solvents are hazardous waste materials and can burn your skin and eyes.
o Dispose of hazardous materials in appropriate hazardous waste containers.
o If the cleaning fluids are flammable, don’t use high pressure air hoses to blow out the tubes of the heat exchanger.
o Watch out for leaks in the heat exchanger shell. The fluids in the heat exchanger can be dangerous and hot. Leaking fluids could burn someone in the area.
o Clean up any leaks on the floor. Wet or oily floors can be slippery.
Regulations in the OHS Act relating to heat exchangers and pressure vessels.
The OHS states that:
 Every heat exchanger or pressure vessel must be fitted with a nameplate (Manufacturer’s plate) that contains the following information.
o The name of the manufacturer
o The country of origin
o The year of manufacture
o The manufacturer’s serial number
o The name of the make and model of the vessel
o The standard of design
o The maximum permissible operating pressure that it was designed for in Pascal
o The maximum permissible operating temperature
o The capacity in cubic meters
o The mark of an approved inspection authority.
 An approved inspection authority must inspect a heat exchange and pressure vessel internally and externally, as well as check all the controls and indicators after any installation or repair work. The inspector must also perform an hydraulic pressure test to 1,25 times the maximum permissible operating pressure at regular intervals. If there is no corrosion, then the internal inspection and hydraulic pressure test does not need to be done as long as the user of the installation gets written approval from the approved inspection authority.
 No person may carry out internal and external inspections and hydraulic pressure tests, unless he or she has a certificate of registration issued by an organization that has been approved by the chief inspector, such as the resident engineer.
 If any inspection or test reveals any weakness or defect in the installation that could affect the safety of the people in the worksite, the person carrying out the test must give the information to the user of the installation straight away. The user must make sure that the heat exchanger or pressure vessel is not used until the weakness or defect is fixed to the satisfaction of the competent person carrying out the inspection.
 The user of a heat exchanger or pressure vessel must keep a record in the worksite of the results of all the inspection, tests and repairs that are recorded, dated and signed by the competent person.
 The user may not keep a heat exchanger or pressure vessel under pressure unless it is kept clean and free from:
o Carbonized oil or other inflammable material that may catch alight under general working conditions
o Material that may cause corrosion, and
o Material that may react to chemicals and cause an uncontrolled rise in pressure and a possible explosion.