Safety Under Pressure Changes

Jon Lawson

Eike Kobbe introduces a solution designed to keep gas pipelines flowing and enable operators to tackle any pressure changes

The oil & gas industry is a sector where fast reaction times are absolutely non-negotiable.

Across the whole production process, downtime (particularly that which is unplanned) can often be both costly and dangerous.

When it comes to gas pipelines, being able to detect and react to any changes in pressure is mission-critical.

The DVGW (German Technical and Scientific Association for Gas and Water) certified safety shutdown units (SSUs) from Böhmer serve to safeguard gas pipeline systems to the current directives for such units to DIN EN 14382 as complete turnkey units.

Each unit consists of a ball valve with full passageway, a simple action, spring-closing actuator and a gas-high-pressure control with two high pilots and one low pilot.

The spring-loaded actuator closes the SSU when the gas pressure as measured increases to above the maximum permissible pressure that has been set for the pipeline.

In addition, it keeps the ball valve closed if the gas pressure as measured is below the minimum that’s set on the low pilot. This prevents the situation where the SSU remains in an undefined intermediate position when the system is being run up.

Design characteristics

The trunnion-mounted ball valve with two floating, spring-supported PMSS seals is characterised by low torques.

The ball valve is almost without pressure loss and is piggable without restriction. It is also provided with a double-block and bleed function directly on the housing. The unit is suitable for aboveground and sub-surface mounting.

The two first trunnion seals can be replaced easily under pressure in the closed position.

The gearbox of the actuator is watertight and maintenance-free. The actuating forces are absorbed linearly and without torsion in the nickel-coated, fully welded cylinders. The piping and unions are in stainless steel.

The compact Scotch-yoke design of the actuator permits the gearbox to consist of just two housing halves, which are divided horizontally in the middle. The housing halves are of high-strength grey cast iron and are manufactured so that they can hold the actuator lever and the actuator cylinders at the same time without additional unions.

Special sealing agents are inserted when the housing halves are put together. These seal the gearboxes hermetically and prevent the ingress of water and air. An excess-pressure safeguard in the housing prevents an impermissible increase in pressure in the case of overheating caused externally.

In addition, the gearbox is completely filled with a special fluid grease that ensures that all the mechanical actuating parts are lubricated optimally and protected against corrosion. Further advantages of this special fluid grease are that the unit is maintenance-free and that its high level of efficiency is also retained following extended down-times.

The control components are accommodated in a stainless steel cabinet and are thereby protected against external environmental factors.

The main components of the control device are executed in leakage-free control block technology. There are two redundantly arranged high pilots and one low pilot.

A reconnection device permits the high pilots to be activated under line pressure. For setting the pilots on site, the control device has an integrated test device. Two high-pressure rapid venting devices arranged redundantly ensure rapid closing (≤ 5 seconds.). All the components of the control are executed in corrosion-resistant form.

The control and main valves are executed as leakage-free seat valves in block mode of construction, which permits them to be exchanged simply and rapidly. The piping in the cabinet, which is kept to the minimum necessary, is of stainless steel with Swagelok unions.

Mode of functioning

The SSU opens automatically when both of the rapid venting devices of the control unit are connected through, all the control pilots (high and low) are activated and the gas pressure is adequate.

If a pilot trips, the gas pressure in the control line is released and the two rapid venting devices are switched over with the aid of the gas pressure present in the cylinder. The gas pressure in the cylinder falls and the SSU is closed by spring pressure.

The SSU closes if the pressure in the measurement line increases to a level above the maximum set value of one or both of the high pilots or if the pressure in the work cylinder of the actuator falls to a value below the minimum set value of the low pilot.

All the control pilots have a mechanical restart interlock that prevents the SSU opening again in an uncontrolled manner as soon as the pressure as measured has reached the set values of the pilots again. The restart interlock must be unlocked manually on site.

The upstream dryer-filter unit treats the control gas, which is removed directly from the gas line.

A pressure regulator limits the system pressure and therefore the actuator’s torque. The system pressure upstream and downstream of the unit is displayed on two pressure gauges. Two safety valves arranged in parallel are integrated in the control to safeguard the system against impermissibly high excess pressure.

The test device permits the high pilots and their settings to be checked regularly. The check valve in it ensures that the maximum permissible line pressure is confined to the high pilots.

The test valve and the pressure line to the high pilots can be depressurised with the spring-return manual valves.

For maintenance work the manual valve is closed and the gas pressure in the control is released by manually actuating the low pilot. When the pressure has fallen below the minimum set value of the low pilot, the control valve of the low pilot switches and the SSU is closed by spring pressure. When the maintenance work has been completed, the manual valve must be brought back into its open position.

The SSU moves to the position ‘On’ again as soon as the regulated system pressure has reached at least the low pilot set value. To check the system the SSU can be supplied with nitrogen via an N2 connection.

Eike Kobbe is with Böhmer

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