Hydraulic bolt tensioners are used to stretch a bolt in a controlled way so the nut can be seated before the hydraulic pressure is released. The pressure reading matters, but it is not the only thing that determines the load left in the bolt afterward.

The joint itself, the condition of the equipment, and the way the tensioning cycle is carried out can all change the final result.

• Material Differences Across the Joint
• Bolt, nut, flange, and gasket materials: These components do not all react to load the same way. Surface finish, compression, and elastic behavior can affect how much preload remains after hydraulic pressure is released.
• Gasket behavior: One area of the gasket may compress farther than the rest as the joint seats, which can leave that location with a lower residual bolt load.
• Component condition: Damaged threads or a seating face that is not flat can show up as a nut that does not run down or seat normally.

Dimensions Matter Too

• A rough or damaged thread can make one nut run down differently from the others.
• Check the seating face closely. If the nut is not sitting flat, the final load can be off even when the hydraulic pressure was correct.
• Gasket thickness is worth noting too, especially if one section compresses more than the rest of the joint.
• Variations between neighboring bolting locations may produce different residual loads even when the same hydraulic pressure is used.

This is one reason the joint should be treated as a complete system rather than looking only at the tensioner pressure.

Operator Technique Can Change the Result

The tensioning sequence matters, especially when several bolts are being worked together.

A consistent procedure helps keep one bolt from being treated differently from the next. Pressure should be applied and released according to the specified procedure, and the required tightening or tensioning sequence should be followed for that joint.

When several bolts are tensioned at once, the order of work becomes even more important because loading one area can affect another.

Some Load Is Lost During Transfer

The hydraulic load applied by the tensioner is not necessarily the exact load that remains in the bolt after the tool is removed.

Some load can be lost as the nut seats and the joint settles. That is why the required hydraulic load, residual bolt load, and any load-transfer allowance need to come from the approved procedure or engineering requirements for the joint.

Do not assume that the pressure shown on the gauge translates directly into the final retained bolt load without accounting for the tensioning procedure being used.

Keep the Pressure Gauge in Calibration

The gauge is one of the operator’s main references during tensioning.

If the gauge is reading incorrectly, the pressure being applied may not be the pressure the operator thinks is being applied. That error can carry through an entire group of bolts if it is not caught.

Gauge calibration should follow the interval required by the company’s quality system, the equipment manufacturer, and any applicable job requirements.

Equipment Condition Shows Up in the Cycle

A tensioner with worn seals, damaged components, or leaking hydraulic connections may not behave the same way it did when it was in good condition.

Look at the complete system:

• Tensioner body and puller components.
• Seals and collars.
• Hydraulic hoses and fittings.
• Pump condition.
• Pressure gauge condition.
• Connections between the pump and tensioner.

At target pressure, the gauge needle should settle. If it keeps creeping backward, the system is not holding pressure as it should.

Quick Troubleshooting Checks

• When one bolt comes back low and the others are close, look at that location first. A nut sitting crooked on the seating face can be enough to change the result.
• A falling gauge reading during the cycle may point to leakage or another hydraulic issue that needs to be investigated.
• Damaged threads or poor nut seating can change the result even when the hydraulic system is working correctly.
• Critical joints may require another inspection after initial seating or after the equipment has reached operating temperature or pressure, depending on the procedure being followed.

Maintenance and Verification

Regular inspection of the tensioners, hoses, pumps, and gauges helps catch problems before they show up during a bolting job.

Keep service and calibration records with the equipment so there is a history to refer back to if performance changes. If a hose, fitting, seal, or other component is damaged, take care of it before the next critical tensioning cycle rather than trying to work around it.

For more on calibration frequency and documentation, see:

What is the difference between calibration and certification

How often should you calibrate and/or certify your torque tool

Practical Operator Checklist

• Check the bolt and nut condition before installing the tensioner.
• Make sure the threads are clean and lubricated as required by the job procedure.
• Look over the gasket and seating surfaces for anything that could affect how the joint closes.
• Verify that the pump and pressure gauge are ready for service before beginning critical work.
• Follow the specified tensioning sequence instead of changing the order in the field.
• Apply and release pressure in the manner called for by the procedure.
• Record the pressure used and anything unusual that happens during the cycle.
• When several bolts are being tensioned, follow the pattern specified for that joint rather than working around it in an improvised order.

Further Reading

For more on dimensional and manufacturing tolerances that affect bolting tools, see What are the accepted tolerances for Hydraulic Torque tools.

For practical guidance on gauge and tool calibration, see How often should you calibrate and/or certify your torque tool and What is the difference between calibration and certification.

For more on tensioner applications, including subsea work, see The bottom line advantage of Subsea Tensioners.

If you have questions about Hydraulic Bolt Tensioners or want to discuss a specific application, contact David Shaw at david@winshawllc.com or 832-540-3831.

(Guidelines for the Management of Bolted Joints for Pressurized Systems, 2007)