Velocity Control of Pigs

Introduction

Control of pig velocity in low pressures gas pipelines is difficult, leading to inefficient pigging, potential loss of inspection data and possible dangers due to high accelerations. For example:

• Confirmation of the dryness of a newly dewatered pipeline. This is traditionally done by using dry air at low pressure
• Inspection of pipelines when the gas pressure is low. High accelerations could mean that the pig velocity exceeds the maximum recommended inspection speed
• Removal of liquid from a pipeline either during operation or during dewatering. Here the efficiency of the pig is compromised by the high accelerations leading to aqua-plaining and liquid subsequently left behind in the line

In addition, there is the inherent danger of accelerating heavy pigs to high velocities.

Basic Design Concept

The figure shows the basic design concept, detailing one seal on the pig.

The higher, driving pressure behind the pig acts on the outside of the seals, pushing the oversized seal away from the pipewall. Therefore, it acts to reduce friction. Low pressure from in front of the pig is channelled into the centre of the seal. The seal works in the opposite way to a typical pig seal, i.e. it is not a self-acting seal (See below, The trouble with Self-Acting Seals).

The pig would work as follows:

1. When the pig is stationary there is an interference between pig seal and the pipeline and therefore a frictional resistance to motion
2. On start-up, higher pressure (The red arrows) acts to destroy this seal to a certain extent and so reduce the differential pressure required to start the pig. The less pressure required to initiate the pig the better as this is potential energy which is latterly converted to kinetic energy. This is the underlying cause of velocity excursions
3. On acceleration, pig differential pressure drops with increasing velocity (See Pig Differential Pressure Vs Velocity graph below). Therefore, the pressure behind the pig reduces and essentially the ‘brakes’ come on as the seal expands again, increasing friction
4. If the acceleration is violent then the gas downstream of the pig could be compressed such that the pressure in front of the pig (Green) is greater than the drive pressure (Red). Since the ‘low’, Green pressure is channeled into the seals, they expand again and act as a brake.

Note that this is merely a concept and not a detailed design. We are not discussing the feasibility, just the idea. The nice thing here is that there are no moving parts. It is hoped that the seal will act as a control system using differential pressure as its controlling parameter: –

This is an example of how the energy behind the pig, differential pressure could be used to our advantage.

The trouble with self acting seals

Most of the time, Self-acting seals are more than satisfactory for use with pipeline pigs as they are very efficient at sealing. The problem with self-acting seals, which nearly all pig seals utilise, is that they store the energy used to drive them as potential energy. This increases the force on the wall, which in turn increase friction, which in turn increases differential pressure and so on. High Potential Energy behind the pig results, see figure. This is then converted to Kinetic Energy, which results in high velocities and accelerations.

Another characteristic of the seal works against us as well – the Pig Differential Pressure (DP) against velocity curve. The figure below shows a typical DP vs pig velocity curve. As the pig velocity increases, then the DP or friction reduces. Therefore, the store of potential energy behind the pig must be dissipated as the pig speeds up.

This concept is an attempt to redress this problem by replacing the seals with non Self-acting seals.

Simulation

Now we can use our model of compressible pigs, Piglab-Compressible (See Software) to simulate the motion of such a pig and compare it with a similar bidi pig. A model of the pig friction against differential pressure has been set up and used in the analysis to simulate the new seal type. The analysis is based on a straight length of line initially with a friction coefficient of 0.9, and then with a sudden change in friction to 0.6. The next graph shows the velocity against time output for both the normal bypass pig and the VC Pig (Velocity Control Pig), with both pigs having a drive Differential of 1.1bars in the first section of pipe. This shows that a drop in peak velocity of about 40%.

However, it is possible that the VC pig will have a lower drive pressure anyway for a given situation.

References

1. G. Smith “Pigging Velocity and variable pig speed”, Pipeline Pigiing and Integrity Monitoring Conference, 28th Sept to 2nd Oct 1992, Amsterdam
2. D.C Hipple, W.C O’Neill, “Test of Cryogenic Pigs for use in Liquidifed Gas Pipelines”, September 9, 1982
3. J.L Cordell “The latest developments in pipeline pigging world wide”, Pipe and Pipeline International, July-Aug 1994
4. J.M.M Out “On the dynamics of pig-slug trains in gas pipelines”, 1993 OMAE-Volume V., Pipeline Tech ASME, 1993
5. L Mathews, M Kennard, A O’Donoghue, Pig Velocity Control,Pipeline Pigging Conference, Amsterdam 1995