Cost Justification... Page 2

 

COST JUSTIFICATION:
INITIAL PURCHASE VS. ALTERNATIVES

On new equipment and retrofits, Multi-Jackbolt Tensioners are generally more expensive than standard nuts/bolts. However, in many cases, the existing nuts and bolts are of special design or materials. In larger size ranges, MJT’s can be equal 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.

For example, hydraulic wrenching is a commonly used method of torquing large diameter nut/bolts. The initial purchase price of hydraulic wrenches can be tens of thousands of dollars and there are costs of accessories such as adapters, hoses, hydraulic power units and special sockets. Depending on the manufacturer, in addition to that there can be heavy maintenance costs and reliability issues.

For example, a recent job for a heat exchanger had (14) 3-1/4” A193-B7 studs to be tensioned using MJT’s. The cost for these Multi-Jackbolt Tensioners was $5,628.00. This compares to a purchase price of around $17,000 for a dedicated hydraulic wrench. In some instances where there are hundreds of bolts to tighten, hydraulic wrenches may have less of a cost impact ignoring other factors to be discussed later.

Another alternative method is hydraulic tensioning. Hydraulic tensioners are used as a tool to stretch a bolt but are removed once bolt tensioning has been accomplished. Hydraulic tensioners require longer studs that sit above the standard nut. If not set up for these, new studs must be purchased to utilize the system. The unit cost of hydraulic tensioners is generally much more expensive than the total cost of Multi-Jackbolt Tensioners. Accessories must be purchased, such as hoses, fittings and power units. The tensioner itself, however, can be moved from stud to stud if time is not a factor. Multiple hydraulic tensioners can be ganged together but there can be heavy initial cost depending on the number of units used. The tensioners rely on seals that can be prone to failure.

Bolt heaters are another alternative bolting method. Depending on the design, these units can be very expensive to purchase. They are very slow, have limits in tensioning power and accuracy, and, if used carelessly, can be hazardous to handle.

 

SAFETY

Aside from the moral implications of providing for worker safety, organizations must be aware of the tremendous costs due to injuries on the job. Because Multi-jackbolt Tensioners require only small hand or air tools, they are arguably the safest bolting method for tightening large diameter bolts/studs. The sledgehammer is still probably the most dominant tool when attempting to bolt up a piece of equipment. The brute force method is prone to hand, arm, leg, face, and back injuries. For example, the piston rod to crosshead jamnut connection on a reciprocating compressor is such that tightening methods as described above cannot be utilized. The large nut is accessed through an inspection door and can be turned using a large, oversize wrench hooked to an overhead crane (Fig. 7), floor jack or most commonly, it is struck with a sledgehammer.

In one real world example, a worker was using this method on a 2-1/2” piston rod when the wrench dislodged from the nut. The wrench then swung up, striking the worker in the face. The worker was put on disability for the good part of a year. Subsequently, (36) Multi-Jackbolt Tensioners, similar to Figure 8, were installed with no further injuries.

Hydraulic wrenches also have safety concerns, as they are high-energy tools. Specific instances of injuries are hand and arm injuries due to sockets exploding or reaction bars pinching or rotating under high pressure. Hydraulic pressure in these units is usually 10,000 psi or higher and instances have occurred where a hose has let go and injected a worker with poisonous hydraulic fluid causing severe injury. Figure 9 shows a photo of sockets that blew apart on a turbine deck, narrowly missing the personnel working in the area. Additionally, large wrenches can also be very heavy and they must be moved into place to function. This heavy lifting has contributed to back and other injuries.

Bolt heating has high-energy voltage requirements and also is a source of injuries mainly related to burns. There have been instances where the heaters have ignited flammable liquids nearby, causing major fires. This is especially a concern where hydrocarbons are involved.

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