The use of hydraulic nuts for compressor crosshead applications

Introduction

Reciprocating compressors are widely used in oil and gas refinery facilities. They constitute an important part of the mechanical equipment in the oil and gas industries and are used for different applications in the main and auxiliary process cycles.

As an important part of the process, companies strive to keep the compressors working efficiently and so their maintenance is extremely important. Bolt tightening specialist and hydraulic nut manufacturer Tentec has developed a solution that will not only retrofit onto existing machines in minutes, rather than the hours it takes to fit a torque activated type nut, but will also increase the time between maintenance and therefore save costs.

The importance of maintaining the crosshead joint in a compressor

Figure 1 shows a cross section of a reciprocation compressor used in the hydrocarbon market. Of particular interest to those responsible for maintaining the integrity of the compressor and ensuring that it is both safe and efficient to use is the piston rod and crosshead jam nut on the left hand side of the diagram.

Taking a closer look at the joint, Figure 2 shows a detailed cross section of the piston. The nuts are used at the crosshead joint and it is here that the piston rod is locked to the crosshead to prevent rotation. A technical manager at a leading compressor manufacturing company has described this as “the most vital connection on a reciprocating compressor”. This is because if it is not maintained properly or the jam nuts adequately replaced, then the result is piston rod fatigue failure and work stops while the compressor is decommissioned. Figure 3 shows failed piston rods on a compressor.

Figure 3. Crosshead joint failure on reciporcating compressors.

Important factors in preventing crosshead joint failure

The best analogy is to consider that the piston rod is effectively a bolt and the bolt and joint can be considered to act like a very stiff spring. The bolt, when loaded, induces a clamping force and spring theory (Hooke’s Law) can then be applied. This states that: “extension is directly proportional to the applied load”.

The joint is clamped together by a tightened bolt and it is this tightening that is critical. For a joint to keep its integrity, one needs to look at the effective length of the bolt, in this case the piston rod.

German Bolting Standard VDI 2330 recommends that for a dynamically loaded bolted joint, the bolt diameter to effective length ratio should be a minimum of 6:1. However, most compressor piston rod to crosshead joints have a ratio of only 1:1.

API618 Standard states that, as a minimum, the manufacturers tightening procedure must ensure a minimum thread root prestress level of 1.5 times the rod’s stress at maximum allowable continuous rod loading. Hydraulic or thermal methods are preferred for tightening piston rod nuts when the rod diameter is 3 in. or larger.

Using hydraulic nuts on the crosshead joints

Hydraulic pressure is applied to the hydraulic nut and the bolt is accurately stretched. Bolt load is proportional to the pressure applied. Once the required pressure is reached, the pressure is held. As the bolt stretches, a gap will appear below the hydraulic nuts locking collar. The locking collar is rotated down against the hydraulic nut body to ‘trap’ the bolt stretch. Hydraulic pressure is released, the bolt elongation is retained and the joint is accurately loaded.

It is this loading that is essential to the integrity of the joint. If too much load is applied, the piston rod will yield and the threads will strip, too little and there will be premature piston rod fatigue failure. Conventional tightening methods such as slugging wrenches or chain and cranes have no control over the applied load and so raise serious health and safety issues.

Some companies use non-hydraulic powered nuts where the axial bolt load is generated by applying torque to ring of jack bolts. In order to avoid applying a disastrous bending moment to the cross head joint these torque activated type nuts have a complex and very time consuming jack bolt cross tightening procedure.

Tentec’s engineers have recently completed a job at a refinery in the US. Tentec’s 4.5 in. hydraulic nuts were successfully retro fitted on a reciprocating compressor, replacing previously used torque activated type nuts. The hydraulic nuts pressurised easily and a total piston movement of 0.022 in. was measured that relaxed back to .015 in. Once fitted on the compressor, loading was completed in minutes, as opposed to the 2 hours that it usually takes using the torque activated type nuts. Maintenance engineers at the refinery were very pleased with the result and the speed of the whole procedure. Tentec has received more orders and all the compressors will eventually be fitted with Tentec’s hydraulic nuts. Figures 4 and 5 show Tentec’s hydraulic nut on a reciprocating compressor in a refinery in the US.

Figure 4. Tentec’s hydraulic nut on a reciprocating compressor in a refinery in the US.

Tentec has supplied hydraulic nuts for compressors to many of the world’s leading energy companies, either off the shelf or customised designs for particular applications. Its engineers are highly qualified and can provide an onsite service to either install the nuts or train personnel to fit the hydraulic nuts accurately, safely and efficiently.

Written by Roy Sheldon, Sales Director, Tentec.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/02062015/the-use-of-hydraulic-nuts-for-compressor-crosshead-applications-090/