Asset Integrity Management Systems: Pipeline Provers – Traditional Vs Compact

Pipeline Provers are used in a custody transfer system to ensure that the system provides the most accurate measurement possible. It does so by offering a station­ary or mobile means to facilitate calibration of flow meters before, during, or after a transfer. As meter performance can be impacted by a wide verity of factors, measurement systems must be regularly checked. The selection of a reliable Prover is therefore vitally important to ensuring the integrity of the system.

By Gobind Khiani – Consulting Fellow-Piping/Pipelines

Flow meter accuracy is always a concern in the custody transfer process. Ensur­ing and maintaining the most accurate measurements is essential for profitable, efficient operations, and fair a transac­tion to both parties. It is therefore typi­cally deemed essential that a Prover be present to verify the meter’s accuracy. While users have a range of Prover op­tions to choose from, in terms of types, and quality, the annual amount spent on proving/metering has reached roughly USD $500 million.

Outdated Solutions

While there are more cost-efficient op­tions available, it is becoming increas­ingly apparent that low-cost solutions tend to have several inherent weak­nesses. Dated solutions, such as master meters and cans, have become know for their reoccurring uncertainty due to me­ter drift, leaving errors undetected, and issues that arise from having to calibrate with an approved technology.

Industry conventional pipe Provers, or ball Provers, continued to be used without in­cident. However, contractors and systems integrators often ignore the potential for longer-term issues that can arise with maintenance, safety, and repeatabil­ity. Over time these potential drawbacks could result in Provers being deemed an inefficient solution, and poorly adapted for changing process requirements.

Modern alternatives, in the shape of small volume Provers, or compact Provers, are field proven and are widely used in offshore industry where a much smaller footprint and resistance to cor­rosion makes them an attractive solu­tion. The benefit of compact Provers is that, generally speaking, they are more broadly applicable, and may offer a safer, flexible, and efficient operation across multiple applications.

Benefits and Drawbacks

With contractors focused on up-front costs, it is up to owners and operators to evaluate the benefits and weaknesses of each Proving solution. If they do, they must question if pipe Provers continue to be the most suitable application for their plant.

Many contractors remain committed to us­ing ball Provers, despite the fact that they may have to face challenges such as the obstacles associated with their bulkiness i.e. transportation, and installation, as well as long term hidden costs, safety, main­tenance, and repeatability vulnerabilities. More specifically, they require higher life cycle and maintenance costs compared to compact or small volume Provers. The accuracy of the Prover can also be ques­tioned over its period of operation. As the oil production processes becomes dirty and corrosive, the demands on a Prover will continue to rise. While a ball prover is static, its lining cannot be changed, and the solution has a limited turndown ratio. Understanding the factors that will impact a Prover is therefore essential for the selec­tion process.

Prover Parameters

Safety

Regulators in United States, such as Occu­pational Safety and Health Administration (OSHA), are keen to minimize the potential for hazardous liquids or vapors to escape into the environment. Not only does this impact the surrounding environment, but it can also be detrimental to those working in the area.

Depending on the application, a ball Prov­er is inevitably exposed to either the en­vironment, potentially high temperature, and/or dangerous products. To mitigate the risks associated with these conditions, it is necessary to follow the safety protocols designed for ball Provers.

Similarly, environmental and safety con­cerns dictate that provision needs to be made for safe, controlled draining and venting of the system for servicing or stor­age. For products such as naphtha (con­sidered hazardous by the OSHA Standard, 29 CFR 1910.1200), significant precautions are required.

Maintenance

The limitations of pipe provers in terms of operational reliability becomes apparent over time. The challenges that commonly arise are different depending on the ap­plication. The following is an overview of different maintenance practices required based on the application.

4-way diverter valve: The 4-way valve is re­quired to change the travel direction of the displacer sphere (the ball) by switching the direction of flow through the Prover. It can be a consistent source of problems in the field. Maintenance is labor intensive and hampered by the fact that there are only three manufacturers of the valves worldwide. The result is long lead-times (over 60 weeks) and high prices if a re­placement is required.

Ball: The displacement sphere is sensitive to damage from the product if material is not properly selected. Degradation of the ball will create leakage and inaccuracy. Material for its liner must be chosen to be compatible with the liquid metered and ensure it will seal through the operating temperature range. Common materials for bidirectional Provers are neoprene, poly­urethane, Viton, Nitrile rubber, and fluo­rocarbon; unidirectional provers usually use polymer spheres. Suitable materials to withstand some hazardous products may be hard to find, however, and the prod­ucts the Prover works with are subject to change. Additional problems, such as the ball deflating or getting stuck, also result in particularly longer/higher maintenance time and cost.

Linings inside pipe: The pipework of ball Provers are, like the sphere, lined to pre­vent damage to the sphere or piston seals. Linings applied to carbon steel pipe are usually epoxied or phenolic material, and must be chosen carefully; not all non-me­ tallic linings resist aromatic hydrocarbons and some can become thermoplastic at high temperatures. If liners do degrade it may be a significant source of downtime.

Corrosion: Like all pipes, the exterior of the Prover is subject to deterioration as it is a pipe. The size of the ball Prover makes pip­ing made of stainless steel unreasonably costly, so carbon steel is used.

When maintenance for Provers is required, the efficiency with which it can be serviced is hampered by the ball prover designs, which is intended to limit its footprint. The ball Prover is frequently located below the metering sections, making access for maintenance difficult. At the end of its life, therefore, companies face the cost and difficulty of disposing of large amounts of contaminated metal.

Accuracy and Repeatability

Pipe provers are manufactured on a to order basis. Occasionally, to compete in the market of fabrication, manufacturers may sacrifice material integrity or design principals, which could lead to future is­sues. If, for example, the ball is not inflated enough, leakage and poor repeatability re­sult; inflated too much and pressure drops cause problems. Thermal expansion and contraction can cause inconsistencies in measurements, calibration.

Conclusion

No solution is perfect, of course. While there are a number of positive associated with ball Provers, there are several advan­tages associated with small volume Prov­ers as an alternative.

These include:

1. Avoiding thermal expansion.
2. Quicker calibration.
3. Low or little draining required.
4. Fully sealed system, no worker ex-posed to fumes.
5. Safe and less challenging operation.
6. Meets API measurement standards Chapter 4 Section 2.
7. Acceptability growing in industry towards replacement or expansion.
8. Resistant to external corrosion and stress corrosion cracking.
9. Easy to operate.
10. Flexibility in operations.

REFERENCES

1. Mohammed Salim
2. Jim Gray
3. Sharon Marsh
4. Julian Cornick
5. Steve Gwaspari,
6. Additional industry colleagues