Many public utility systems use complex, interdependent networks of pipelines, valves, pumps, and other equipment to move water around, making them vulnerable to damage from hydraulic transients, which can occur whenever the flow of a liquid in a pipeline changes quickly — as when a valve rapidly opens or closes or a pump starts or stops.
Flowing liquids have kinetic energy. When they run into a valve that closes or a pump that stops suddenly, the resulting transient shock wave travels back and forth in the pipe until friction or damping controls dissipate it. The pressure also might spike high enough — even for an instant — to weaken and eventually burst the pipe. Similarly, opening a valve or stopping a pump can cause a low-pressure wave that can vaporize the liquid in part of the line, potentially causing negative pressure that can also result in pipeline collapse. The same forces can occur in an unplanned event such as a power outage, so that a pump stops unexpectedly and normal operational controls may not function.
The problem has come to be called “water hammer,” since it often results in a loud banging of the pipe. This sound can occur far from the actual source of the original pressure change, such as a valve.
Even when transient pressures are small or change gradually, regularly occurring fluctuations in pressure can eventually wear out pipeline systems and components. The result can be a costly shutdown while repairs are made. Serious amounts of water may be lost, or sewage can seep into the soil and groundwater. In systems in which water supply lines and sewer lines lie close to each other, breaks caused by hydraulic transients can cause sewage or stormwater to migrate into the water supply.
TO DESIGN OR NOT
Many utilities believe that they can deal sufficiently with the hydraulic transient problem in the design stage. Using complex formulas and computer models, designers determine whether a given design will successfully manage hydraulic transients. Their calculations include the metallurgy or material properties; pipe diameter and wall thickness; the pumps, valves, and other equipment; and the nature of the fluid being pumped.
That type of computer-based analysis can go a long way in managing the hydraulic-transient problem, and it is far less costly to fix a problem at the design stage than it is to fix it after the system is operating.
However, because most utility systems have valves and pumps interacting with each other it is often not practical or possible to foresee all possible pressures that may result. System failures may occur due to incorrect supply or installation of control devices; improper operation of the system so that resulting surge pressures are not adequately limited by the transient control system; or inadequacies in supplied components.
Typically, systems for managing transient pressures and surges are designed by a transient specialist engineer. Design codes offer little explicit guidance about how to appropriately involve the transient specialist.
In many cases, that work is subcontracted out and is performed only after the basic design is complete. So the transient specialist is expected to work within the broad strokes of the existing design, including pipeline material, diameter, strength, pump specifications, and other details. Accordingly, the system may not be optimized for hydraulic transient management.
It would be better to bring the specialist engineer into the process earlier — at the conceptual design, or even the permitting stage — to assess the relative merits of a given facility design.
The specialist should also be involved later in the process, at the commissioning and acceptance stages, to help with any problems that crop up regarding hydraulic transients. That is important because success in commissioning depends on a number of factors that cannot always be verified ahead of time, including adequacy of safety factors inherent in the design codes and manufacturers' materials; the correct specification and supply of surge control devices; correct installation of all transient control devices; and correct operation of the system.
It is much better to check the entire system at the commissioning stage to see if it is operating as it should. The tests required are relatively simple, consisting of attaching a small, high-frequency pressure transducer and data-logging system, usually at the pump station, to record system transient pressures during a series of pump operation tests, including regular pump operation (startup and shutdown) as well as critical cases like power failure, which is simulated by tripping individual or multiple pumps. Recorded pressures from those events is then compared with the transient design predictions to determine whether the system is operating in accordance with its design.
Up to 10% of all pipelines experience some degree of failure, at least in not meeting the required transient performance criteria as developed during the design, during or soon after the commissioning.
Half of these pipelines likely will experience a physical failure as well, although there are generally more factors at work than just transient pressures. Usually, a combination of factors, such as defective materials or improper installation, causes the failure.
ONGOING TESTING IS NECESSARY
Testing for hydraulic transient management effectiveness should continue through the system's lifespan. Equipment wears out. Pumps lose their effectiveness and need repair or replacement — possibly by a more powerful model that will put a strain on pipe that was not expected in the original design. Buildup inside some lines may restrict flow. Metal fatigue from recurrent hydraulic transients can gradually wear systems down until there is a failure.
A regular program of integrity testing for hydraulic transient management effectiveness can be money and time well spent. It can mean early detection of problems before they lead to failure and help optimize the performance of a long-running system. It can also reduce the severity, frequency, and cost of transient-induced or associated failures.
The frequency of such testing may vary from system to system depending on complexity of components, rates of wear and tear on components (which themselves may be influenced by the severity and frequency of transient pressures), and other factors. Generally, an annual program is performed to monitor the regular operation of the system by a high-frequency pressure transducer and data-logging system for one week to one month.
Collected data is reviewed to identify abnormalities and trends in the system, transient pressures, and behavior of control-system components. This suffices for most systems to provide early indication or detection of important changes in the system condition or control device settings and departures from baseline performance. If the monitoring results indicate that the system is robust, the frequency of monitoring may be reduced accordingly. An annual budget of $10,000 to $20,000 would suffice for a typical pumping and pipeline installation, although costs will vary depending on the size and complexity of a system and whether multiple monitoring sites are needed.
Hydraulic transients are a serious issue for public utilities, but they can be managed through good design, transient pressure monitoring, and appropriate specialist expertise.
TRUMPET THE BENEFITS
For a cash-strapped municipality the idea of spending money on a new set of tests may not be warmly welcomed at first. To demonstrate the value of hydraulic transient testing, it helps to couch the idea in terms of wise stewardship over the municipality's infrastructure. Testing prior to acceptance of the system from the contractor can help reduce costs, as it can find out if the hydraulic-transient management components installed will actually do the job.
Designers sometimes add components such as valves that are unnecessary, and at $5,000 to $10,000 for a valve, costs add up. Ongoing testing after acceptance can provide warning of problems such as burst pipe, damaged valves, water loss, treated-water contamination through the inflow of fluid from nearby sewage lines, and leakage of sewage into the ground-water. Managing transients effectively in potable water systems means improved water quality, as there is less propensity for bio-films on the insides of pipelines to be dislodged and particulates carried along in the water flow.
It also means that any identified developing problems are addressed with fewer costs and in a controlled environment rather than when a water main breaks during morning drive time and a roadway is flooded. By managing the risk of catastrophic breaks, infrastructure managers' lives are less disturbed by phone calls from irate taxpayers.
Hydraulic transient testing is a relatively low-cost form of insurance considering both the magnitude and likelihood of a major problem developing.
— McInnis, professional engineer, Ph.D., has more than 25 years of experience providing support for liquid pipeline systems including transient analysis software development, advanced hydraulic analysis and design of surge controls, pipeline route planning, optimization, and risk management. He is an associate and senior water supply engineer in the Calgary office of Golder Associates Ltd.