Collar Bolts in Shell and Tube Heat Exchangers

Collar bolts are used for removable bundle heat exchangers to hold the bundle in place and remove the channel without interrupting or breaking the seal between the tubesheet and shell.

This article concludes the technical discussions and debates on the use of the collar bolts in shell and tube heat exchangers on a previous linked-in post. Link for the post can be found at the reference list of this article. 

Standards and References Quotes 

In the new tenth edition of TEMA standard which recently published in 2019, recommendation for the use of the collar bolts in the removable bundles with B-Type bonnet are added under part RCB-11.8 as summarized below and as per the recommended configuration shown in Figure-1.

When specified by the purchaser, collar stud shall be used on units wit removable tube bundles. Collar stud are recommended for B-Type bonnets. The OD of the stationary tubesheet shall match the mating flange OD, and shall be through-bolted. Every fourth stud in the bolt circle (with minimum 4) shall be a collar type I for type II as shown in Figure 1. 

Collar bolts are only used to maintain the gasket integrity and position when the channel is removed and torqued prior to pressurizing.

As an alternate to collar studs, every fourth bolt ole in the tubesheet may be drilled and tapped to size of the stud bolt. The studs in the threaded holes shall be double nutted on the shell side or provided with machined flats to allow of the tube side nut without rotate the stud.

In API 660, Para 7.5.2.4: A full diameter stationary tubesheet shall be provided for removable tube bundle exchangers with bonnets (see TEMA, Figure N-1.2, Type B stationary head, quoted in Figure-2). The tubesheet shall be provided with collar studs or tapped tubesheet holes for a minimum of 25 % of the bolts (4 minimum). The tubesheet design shall allow for hydrostatic testing of the shell side with the bonnet removed and all bolting installed.

If collar bolts or drilled-and tapped holes are used, at least four shall be provided and their location shall be identified on the drawings and by stamped markings on the OD of the tubesheet.

In PIP VEFV1100M Vessel/S&T Heat Exchanger Standard Details, standard configuration for collar bolts dimensions are provided as shown in Figure-3 

Surface condensers are design to the requirements of HEI (Heat Exchange Institute) per the typical configuration shown in Figure-4, in which both tubesheet are fixed and the gasket between the tubesheet and shell flanges cannot be attended without the removal of one of the tubesheets.

The following alert is quoted from the manual of one of the famous and reputable surface condensers manufacturers in the world:

“It is important not to break this seal between tubesheet and shell flange. The tubes are expanded into each tubesheet holding them firmly in place, and the shell seal cannot be replaced without retubing the entire condenser. To prevent breaking the joint, it is important that all nuts be removed from the waterbox flange side and not from the shell flange side. Do not loosen or remove the stake studs and double nuts on the shell side.”

Having a leak from the shell side would be of a great impact to the plant as it will break the vacuum. In such configuration, the use of the collar bolts might be a must. 

Despite of the illustrated advantages for the use of the collar bolts, there is a debate about the disadvantages as it might be cause for troubles instead of enhancing the exchangers maintainability. In the following section the main prose and cons as for the use f the collar bolts are summarized .

Pros for the use of collar bolts 

The main objective and advantage of the collar bolts is the better maintainability considering that each time the channel removed, the bundle shall be removed for replacing the gasket between the tubesheet and shell to avoid leakage after pressurizing the exchanger. Bundle gasket replacement is time consuming and increases MTTR (Mean Time To Repair/Restore). 

Cons for the use of the collar bolts

1.      The opposite opinion for the use of the collar bolts is that it is nonmandatory to remove the bundle if the channel removed. This opinion is built on some special experience in using kamprofile gaskets with graphite APX-2 and the application of initial proper stress and following recommended torque to reach the desired gasket stress. This experience was explained by Mr. Albert De Sousa, P. Eng as it can be reached in the comments of the linked-in post provided in the reference list.

2.      In case of using tapped holes and the bolts get stuck and the attendance for the holes machining and replacement of the bolts would be a time consuming and might be beyond the readiness of the maintenance crew for the task.

3.      Inadequate marking or stamping of the collar bolts might mislead the maintenance crew, and the collar bolts can be removed by mistake

4.      Relative higher cost due to the bigger tubesheet size and the aching required for the blots holes in the tubesheet. 

References: 

[1] Linked-in post https://www.dhirubhai.net/posts/activity-6600090101194211328-e7PX

[2] API Std 660 - Shell-and-Tube Heat Exchangers

[3] TEMA Tenth Edition, 2019 (Standards Of The Tubular Exchanger Manufacturers Association)

[4] PIP VEFV1100M Vessel/S&T Heat Exchanger Standard Details

[5] Explore The World Of Piping – EWP https://www.wermac.org/equipment/collarbolt.html)