Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners

by Allan Steinbock, VP Superbolt, Inc., Carnegie, PA

INTRODUCTION:

Multi-Jackbolt Tensioners are bolt tightening devices that have been around for over a decade. They are widely used on critical equipment in most heavy industries including Petrochemical. Although the concept is relatively simple, these tensioners have many benefits that are not commonly known. They have proven to be economical in terms of cost and in terms of equipment reliability, which will be the focus of this paper.

DESCRIPTION OF SYSTEM:
Multi-Jackbolt Tensioners (MJT) come in many variations including nut style, thrust collars (unthreaded) or bolt style devices. Due to the wide variety of components available, the MJT system can be retrofitted into the same area as a standard OEM nut or bolt. This paper focuses on the nut style tensioner (FIG. 1) as this is the most commonly used. However, most of the benefits are applicable to the other designs.

The MJT system is composed of a round nut body with an internal thread identical to a standard fastener. In between the thread and the outside diameter is a series of drilled and tapped holes designed to accept hardened jackbolts that pass through the entire nut body. A hardened washer is always used under the jackbolts to protect the bearing area of the equipment that is being worked on (FIG. 2).

To apply the system, the hardened washer is placed over the existing stud, bolt, rod or shaft to be tightened. The nut body is then threaded onto the main thread of the standard fastener hand tight against the washer. The tightening torque is applied to the individual jackbolts with a standard hand held torque wrench or air tool. Turning the jackbolts creates a thrusting of the nut body away from the washer surface, creating bolt tension and imparting a stretch on the main thread (FIG 3 & 4).

MJT’s stay in place and remain on the equipment until removal for the next outage. An equivalent torque on a standard fastener can be achieved with a fraction of the torque input. For example, to pre-stress a 4”-8tpi bolt to 45,000 psi (520,650 lb. of preload) you would need to torque a standard nut to 30,650 ft-lb. With a standard MT series MJT, the same prestress can be achieved with only 190 ft•lb on each jackbolt. Figures 5 & 6 show torque values that indicate the dramatic mechanical advantage of MJT’s compared to standard hex nut torque values. The system has been applied to numerous applications common to Petrochemical plant on turbines, pumps, heat exchangers, piping, valves, reactor vessels, centrifugal compressors. On reciprocating compressors they are used on the piston rod, crosshead connection, piston end nut, anchor bolt, doghouse bolting, connecting rods, power cylinder and more. It should be pointed out at this point that different applications might have different installation and removal procedures and that it is critical to follow the correct instructions for a given product.


Since applying Multi-Jackbolt Tensioners as an alternative bolting method is new to some people, cost justification and reliability issues come to the forefront when compared to alternative or existing methods. Although many of the equipment reliability issues tie into cost justification, we will attempt to discuss them separately and prove them with actual case histories.

COST JUSTIFICATION:
INITIAL PURCHASE VS. ALTERNATIVES

On new equipment and on retrofits, Multi-Jackbolt Tensioners are generally more expensive than standard nuts/bolts themselves. However, in many cases, the existing nuts and bolts are of special design or materials. In larger size ranges and covered or acorn nuts, MJT’s can be close to or less than original nuts/bolts. For example, on a recent job for a large coupling, (18) 6-13/16” tensioners were required. The initial cost of the tensioners was equivalent in cost to the machined standard nuts. However, in the comparison of MJT’s to alternative tightening methods is where most cost justifications occur.

Fig. 6

At 60000 psi Main Bolt Stress

At Max MJT Capacity

Thread Size (inch)

Main Bolt Load (lb)

Hexnut Torque (lbft)

MJT Torque (lbft)

Main Bolt Load (lb)

Hexnut Torque (lbft)

MJT Torque (lbft)

Main Bolt Stress (psi)

Mechanical Advantage

1-8

33,060

480

18

64,829

941

36

117,656

26

2-8

159,000

4,379

69

349,458

9,624

152

131,870

63

3-8

379,200

15,374

154

764,255

31,002

310

120,926

100

4-8

694,200

37,203

226

955,318

51,197

310

82,355

165

5-8

1,104,000

73,545

427

1,343,185

89,479

520

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