Frequently Asked Questions

1. How are BAHXs tested?

a. A BAHX is first structurally tested. For hydrostatic test methods, each stream is pressurized, with the other streams at zero pressure, to 1.3-times the design pressure per the requirements of ASME. The unit remains pressurized for 5-10 minutes. Pneumatic test methods can be used with the pressure level being adjusted to 1.1-times design pressure.

b. Leak testing is performed after the structural test has been successfully performed. Internal leak testing is performed by pressurizing each stream individually and monitoring small nozzle valves on the unpressurized streams using soap film solution. External leak testing is performed using soap solution on the exchanger external joints.

c. Helium vacuum leak testing may also be used to validate the leak tightness of an exchanger, usually at an extra expense.

2. Are the exchangers repairable if a leak is detected after the exchanger has been put in service?

a. Exchangers are repairable - to what extent depends on the type of leak and where it is located. Leaks develop from a number of sources - freeze failure, corrosion, and thermal fatigue are the most common. When a leak has been detected, a manufacturer of BAHX or qualified field repair company specializing in BAHX repairs should be called to consult and possibly perform the repair. Repairs can include welding shut the leak or plugging of the passage that is leaking.

3. What are the temperature and pressure limits of BAHX?

a. Temperature and pressure limits vary from supplier to supplier. In general, most of the world leaders in the production of BAHX and members of ALPEMA (Aluminum Plate Fin Heat Exchanger Manufacturer’s Association) produce heat exchangers that are capable of pressures from full vacuum to 1400 psi. Most manufacturers’ products are capable of exceeding this pressure limit but to what extent varies greatly between the different suppliers, as does the size of the exchanger. The pressure limit of Chart Energy & Chemical’s BAHX exceeds 2000 psig.

b. Temperature limits are dictated by ASME code and range from -450°F to 400°F. However, practically speaking, it is not desirable to exceed 150°F, especially at elevated pressure (in excess of 200 psi).

4. What aluminum alloys are used and what determines their selection?

a. Economics, availability, and code limitations dictate the type of alloy used. Generally speaking, 3003 alloy is used in the core block (consisting of parting sheets, side/end bars, and fins) and 5083 alloy is used in the header and nozzle components. 6061 alloy is usually the alloy of choice for flanged connections.

5. What type of stainless steel to aluminum coupling is preferred?

a. For mechanical couplings, a RFWN flange is often used. For non-mechanical connections there are a variety of suppliers of stainless to aluminum transition joints. It is best to consult with the BAHX supplier as to what supplier is recommended. Usually the BAHX supplier has experience with each of the suppliers and is capable of making a qualified recommendation as to which supplier(s) are adequate.

6. What process information is required for specification?

a. A BAHX supplier can provide a blank data sheet to be filled in. Stream inlet and outlet temperatures and pressures are needed, as are either the physical properties at the inlet/outlet or the stream chemical composition. Chart Energy & Chemicals prefers to receive a customer’s HYSYS file for the specified exchanger and has a program which converts the HYSYS output to Chart E&C’s design program input.

7. Installation concerns? Piping stress on nozzles?

a. Consult with the supplier’s installation and operating manual. Usually the supplier will have specified methods of lifting and installing the exchanger. The ALPEMA suppliers will perform thermal expansion/contraction mechanical calculations in order to provide adequate piping strength.

8. What are the commissioning requirements and start-up and cool-down rates?

a. There are various considerations required for start-up and shutdown which are covered in the ALPEMA standards. Typically these considerations have to do with limiting thermal stresses imposed on the heat exchanger.

Alpema Standards

 

9. What are the storage conditions?

a. See the ALPEMA standards. General practice is to keep an exchanger to be stored for any length of time in a clean, dry environment away from equipment that could potentially damage the exchanger. BAHXs can be shipped with a nitrogen blanket and gages with valves to monitor the pressure. Damage in storage or shipping can be detected if pressure has been reduced or eliminated from the nitrogen blanket.

10. When is upstream filtration required? Can blockages be removed?

a. Due to the nature of the internal construction of BAHXs it is recommended that filtration almost always be employed. Restrictions can be removed but can be difficult at times to eliminate. Consult the ALPEMA specifications for the options for eliminating blockages. (Back puffing and solvent washing are most typical.)

11. What fin types are available? What is their selection criteria?

a. Fin types come in a wide variety from most of the ALPEMA suppliers. They range in height, thickness, configuration (straight, perforated, herringbone, and serrated), and fin count (fins per inch).

b. Fins are selected on the basis of required design pressure, thermal performance, and allowable pressure drop. The optimum fin is one that does all of these things at the lowest cost.

12. Corrosion and mercury features

a. Chart Energy & Chemicals is the developer and world leader in providing exchangers that are suitable to perform in a mercury environment. Due to the variety of conditions the exchanger may operate in with mercury present, and the proprietary nature of Chart E&C’s design, we recommend that a potential customer speak to us about their individual needs.