Can variable speed drives (VSDs) replace surge vessels on pumping systems?

There is a temptation to avoid the use of air vessels on pumping systems and a plethora of options that are proposed to avoid their use, from surge anticipation valves, through triple acting air valves to variable speed drives. 

Maintaining poorly designed surge vessels can make life difficult for those responsible for ensuring they are compliant and working correctly. In view of financial pressures in the water industry,  poor decisions are being made with respect to air vessels.

This post considers where surge vessels can be practically replaced and where they cannot, and also some of the implications of not using an air vessel on a pumping system.

Potable Water Systems

There is a recent and dangerous trend in some water companies to completely isolate air vessels and rely on VSDs to create a slow pressurisation and depressurisation of networks. 

While there are places where VSDs can replace air vessels, this is rarely if ever an option on potable water systems. Potable water systems have a legal and moral requirement to sustain positive pressures at all times to avoid contamination. 

Air Vessel – Courtesy of Quantum Engineering Developments Ltd

If a potable-water  network were to be controlled by VSD, there is a possibility this would work for much of the time, but a power failure or motor trip, pump failure or operator intervention will generally result in negative pressures that could at least lead to major contamination of the network but could also lead to catastrophic bursts around the network as are increasingly prevalent in water company systems.

As an added complication,  varying pressures on a complex network can lead to unusual burst phenomena under low demand conditions. When pressures drop at the pumping station, this leads to a flow reversal in the network that propagates to the extremities of the network. Any extremity with no demand on it will experience double the pressure drop of the rest of the network. This can lead to pressures dropping sufficiently to allow contamination and/or cavitation and subsequent bursting at the periphery. To sustain the integrity of a network, it is important to avoid this pressure cycling. This cannot be achieved with VSD drives.

For potable networks, VSDs cannot replace the function of a surge vessel and a full pressure surge analysis. 

Note on Transient Loggers

Some water companies may have justified the use of VSD for surge control as a result of the now prevalent transient logging programmes. A surge analysis would identify the majority of potential surge issues, while a transient logging  programme is likely to miss them. The position and speed of loggers paramount in the identification of burst and contamination, and it would be almost impossible to locate a logger correctly and correctly sampled without an associated surge analysis study.

Sewage pumping systems.

For potable water systems, the risk of contamination is significant on any system in which VSDs are proposed to replace vessels. For sewage systems, this is not a consideration. 

A sewage pumping line can be protected from severe negative pressures providing the ramp rate on the pump is slow enough to prevent vapour pressure or column separation. This can only be reliably determined by running a simulation. 

However, the VSD ramped pump stop will again only be effective during normal ramping, and pump trip/failure and power failure incidents will be vulnerable to unprotected pressure surge. The risks associated with this differ from system to system. There are situations in which a pump trip on a VSD controlled system will result in full cavitation. The risks of catastrophic failure associated with such trips are usually prohibitive, but if the full cavitation events are very short-lived, and are on a sewage system in which the sewage is likely to attenuate the severity of the events, and the systems are HDPE which is relatively tolerant of minor cavitation, then it would seem acceptable to take a view on the risks associated with a VSD protected system. Generally for other systems, the risks of catastrophic failure are too great to rely on this mitigation in isolation. 

Industrial systems.

There are a number of industrial systems in which an occasional catastrophic failure can be tolerated because of the nature of the fluid and the installation. It may be that as a result of a detailed pressure surge analysis, a client will consider the risks and take a view, based on the implications of a serious burst for their system. In this instance it might be feasible to use a VSD as the prime surge protection, but only following a detailed surge analysis, and HAZOP investigation.

Cooling networks

Many industrial cooling networks involve very large fluid flows with negligible static lift. These generally cannot be protected by air vessels. A details surge analysis would identify if VSDs could play a part in reducing the risks on a system, but it is unlikely that they would be used as comprehensive surge mitigation.

Conclusions

A dangerous trend to replace air vessels with VSD drives is borne out of ignorance of the pressure surge phenomenon and the implications of pressure surges on a pipe system. While VSDs can reduce the risks on a system, only in exceptional circumstances would it be appropriate to use them as the only solution. It is essential that a full surge analysis is undertaken on all systems with significant hydraulic inertia, and comprehensive mitigation measures are designed and implemented to ensure pipe network safety and to avoid fluid contamination. This is particularly true for potable water network.

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