All physical phenomena is transient in nature and so do rotating equipment follow the same principle. The difference between a good engineer and a great engineer is that - A good engineer thinks how a piece of equipment is meant to be running while a great engineer understands that transient events (unsteady state) occur before a rotating piece of machinery runs the way it is meant to be. For engineers, it is necessary to understand how transient events affect the operation of large Industrial machinery. This will at least ensure you wake up the next day to make a dollar another day! Because if you thought it wasn't necessary, chances are that you'd probably end up in the Boss's office for an early performance review when your machinery conks out.
Pumps are the basic transporters of liquids in an Industrial Plant. They go through the three main stages - Startup, Running, Shutdown. It is during a startup and shutdown, rotating machinery go through an arduous journey of mechanical stress and strain for a while before being able to pump liquids steadily.
The above image is a simple construction of a pump system. It consists of a centrifugal pump with a suction pipe, a check valve that allows liquid to flow in only one direction and a discharge block valve to shut off the flow through the pipe. Pump systems are equipped with controllers that detect whether flow through the piping is blocked and instruct the pump to shutdown. Any abrupt shutdown means you can be sure of the chances that the pump is on its way to becoming damaged.
A liquid hammer can be simply defined as - Any rapid change in liquid flow conditions in a pipe that causes very high pressures and even reversal of flow. It's a phenomena where the flowing liquid in a pipe smashes itself against the pipe walls and valve fittings that the effects manifest visibly as vibrations and physical damage.
Vibrations are bad!! because pipes are placed on supports which also feel the stress and result in pipes bursting, support structures getting damaged and piping joints starting to leak. When pump systems carrying toxic liquids fail spectacularly due to a liquid hammer, its a recipe for potential human casualty as well (and probably the next script for an upcoming Hollywood Director who's aiming for an Academy Nomination!!).
Liquid Hammer is vast topic to deal with and in the interest of keeping this article brief, the Author chooses to describe one of the ways a liquid hammer can be avoided - i.e., By Controlling the shutdown behaviour of the centrifugal pump.
Pumps can trip due to power failure, mechanical failure, human error to name of few. Theoretically, a pump that is running at a certain speed can be assumed to reach a standstill in no time! However practically this is never possible and depending on the pump size and capacity, the pump takes its own sweet time to reach a state of rest. When a pump shuts down instantaneously, it produces the worst effects of the liquid hammer. So the idea to avoid a liquid hammer 'Is to slow down the rate at which a pump decelerates'. The slower a pump decelerates, the higher are the chances of avoiding a liquid hammer.
Engineers don't stop at sizing a pump for an application. They take the time off to estimate the time required for a pump to shutdown. To make a simplistic analysis that is free of any external forces acting on the pump impeller, the kind of data that a Chemical or Mechanical Engineer gathers to estimate the pump speed's decay rate (dN/dt) is the total inertia of the system ('J' expressed in kg.m2), Torque ('T' expressed in N.m), pump speed ('N' expressed in 'revolutions/min' or 'rpm'). All these terms are correlated with each other as shown below to derive an algebraic expression (SI system of units) with which the time taken for a pump to shutdown can be estimated.
To demonstrate how the expression in the blue scroll is used, consider the hypothetical situation with hypothetical data where you were having a Burger during your break, and 30 seconds after you started chomping it down, a 250 kW pump running at 1460 rpm with an inertia of ~11 kg.m2 tripped due to a power failure. Using a simple tool like MS-Excel and the above expression, a graph can be plotted to estimate the time taken for the pump speed to reach a standstill.
Inferences from the above graph,
Note: Pumps shutdown times are also influenced by the fluid resistance, dynamic imbalance, misalignment between shafts, leakage and improper lubrication, skewed bearings, radial or axial rubbing, temperature effects, transfer of system stresses, resonance effect to name a few and therefore in reality, shutdown times can be lower than estimated by the above method.
With the pump shutdown time estimated, Engineers would use these numbers to conduct further studies called "Pipeline Surge Analysis" to determine if a pipe/pipeline operating pressures exceed its design pressure. If the pressure rise caused by the liquid hammer is higher than the design pressure of the pipeline, engineers embark on a pipe/pipeline saving mission whereby one or more of the following remedial measures are chosen
Vijay Sarathy holds a Master’s Degree in Chemical Engineering from Birla Institute of Technology & Science (BITS), Pilani, India and is a Chartered Engineer from the Institution of Chemical Engineers, UK. His expertise over 16 years of professional experience covers Front End Engineering, Process Dynamic Simulation and Subsea/Onshore pipeline flow assurance in the Oil and Gas industry. Vijay has worked as an Upstream Process Engineer with major conglomerates of General Electric, ENI Saipem and Shell.