Appeared in Valve Magazine on October 11, 2016.
Water hammer, high-velocity fluid flow and contaminated hydraulic media are among the worst enemies of high-pressure hydraulic systems. Water hammer, in particular, can be very destructive in high-pressure descaling applications. At steel mills, persistent water hammer in high-pressure piping systems often leads to burst pipes, cracked welds and descale header damage. In some cases, control valves have actually exploded due to water hammer.
This article will discuss the damaging effects of water hammer and how proportional poppet valve technology can eliminate this phenomenon in descaling applications.
Exposing Water Hammer and Its Destructive Effects
Because descaling is a high-pressure, high-flow application, descaling systems are very prone to fluid acceleration and deceleration shock forces. These forces are commonly referred to as water hammer forces or hydraulic transients. The term water hammer describes the loud noise or bang that accompanies the occurrence, which sounds much like a hammer being struck against the hydraulic piping.
Water hammer forces can be very destructive and may lead to:
- Pipes bursting
- Flange connection points cracking
- Pipe supports being pulled off of equipment and walls
- Concrete foundation damage at pipe anchor points
- Valves exploding (literally)
Water hammer is created by the sudden restraint (or release) of a column of water at high pressure. This hammering effect happens because a pressure wave is created by the instantaneous change in fluid velocity when a valve is suddenly closed or opened. The speed at which this pressure wave travels through a given fluid is referred to as the celerity of the fluid. A particular fluid’s celerity is a function of its bulk modulus and its density. The celerity of water that is typically used in descaling systems is approximately 4,719 ft./sec (1,438 m/sec). This pressure wave, which generates the relatively instantaneous water hammer forces, travels the length of the pipeline and returns to the source in a finite time.
Controlling Valve Closure Time to Manage Water Hammer
Controlling valve closure time is one of the lowest cost ways to minimize water hammer. Closing the valve within a period of time that is greater than the time it takes the pressure wave to return to the valve greatly minimizes the effects of water hammer. To do this effectively and still meet the operating parameters of high-production process lines, a variable speed stroke profile is required. This method of operation permits fast closing of the valve up to the point where restriction of flow starts and then slowing the rate of closure to the time required to minimize any water hammer in the system.
One of the best ways to achieve a stroke profile that permits variable opening and closing rates is proportional stroke control. Controlling the valve’s stroke profile in this manner enables a reduction in the valve’s closure time at the position and speed required to diminish water hammer. Until now, proportional stroke control for water-operated descale systems required significant filtration and maintenance to implement and was very unreliable.
New developments in proportional poppet descale valve technology, which incorporate electromechanical actuation, provide the ability to precisely control the metering poppet’s speed and position and can reduce water hammer.
Understanding the Proportional Poppet Valve Design
Mechanically actuated poppet valves contain a metering poppet or disc that extends and retracts in the valve liner. The angled seat on the poppet makes contact with the angled valve seat to shut off flow through the valve. As the poppet is moved by the mechanical actuator, it opens or closes the opening between the metering nose and seat as well as permits flow through the slots in the liner proportionally as the stroke increases. Flow and pressure drop through the valve is determined by the electromechanical actuator’s positioning of the poppet.
Proportional poppet valves that incorporate ceramic seat surfaces can operate over extended periods of time on corrosive media or with high-velocity water that contains some degree of contamination from sand or scale. With this type of valve, the poppet and seat surfaces are constructed of ceramic material for wear and corrosion resistance. To provide metering control of the fluid flow, the poppet utilizes a special metering nose profile for precise control of flow and pressure. Here’s how the metering process works:
- When shifted to the open position, the poppet is retracted in a direction away from the seat to a position where the seat and slots are exposed for full flow—from the inlet port to the outlet port. As fluid enters the inlet port, it travels into the valve body, in through flow slots in the liner and out through the seat to the valve outlet port.
- When the spool is extended in a direction toward the valve seat, the poppet seat contacts the valve seat to stop flow from reaching the outlet.
The proportional stroke control of the linear actuator combined with the metering nose on the poppet mitigate water hammer, as the transition from closed to open is controlled to a rate that meters flow for the initial or final 10–20 percent of the stroke.
Utilizing a variable stroke profile enables one proportional poppet valve to perform multiple operations such as descale header control and header prefill functions. This reduces the need for additional prefill control valves or pressure breakdown orifices, saving plant and maintenance engineers money on extra equipment.
Reducing Annual Operating and Replacement Costs
In most industrial applications, valves are cycling at high rates ranging from several hundred to several thousand cycles per day at pressures ranging from 2,000 to 5,000 psi and flow rates up to 3,000 gpm. In these applications, repeated pressure spikes due to water hammer caused by fast closing valves eventually damages the piping system and auxiliary components.
By replacing fast-closing poppet valves in the descale system with proportional poppet valve technology and eliminating water hammer, companies can save on annual plant operating costs as well as replacement costs for damaged descale headers and pipes.
Mickey Heestand is an ASNT NDT Level III, certified in several NDT methods and oversees Hunt Valve’s welding and nondestructive test procedures, Welder Workmanship Training and Examination and contract engineering reports.