Stopping the “Water Hammer” in Your Hydraulic System
In last week’s email, we talked about decompression and how to control it.
Let’s wrap up the topic of noise by talking about a phenomenon known as ‘water hammer’.
Water hammer is the term used to describe the effect that occurs when the velocity of the fluid moving through a pipe suddenly changes.
Sudden change in fluid velocity causes a pressure wave to propagate within the pipe. Under certain conditions, this pressure wave can create a banging noise, similar to that you would expect to hear when beating a pipe with a hammer. Hence the phrase.
Not surprisingly, common symptoms of this problem are high noise levels, vibration and broken pipes.
When a moving column of fluid hits a solid boundary – when a directional control valve closes suddenly for example, its velocity drops to zero and the fluid column deforms, within the rigid cross-sectional area of the pipe, to absorb the energy associated with its motion – similar to a car hitting a concrete wall.
However unlike a car, the fluid is almost incompressible so the deformation is small and a store of energy accumulates in the fluid – similar to compression of a spring.
The magnitude of the pressure rise that results from the subsequent release of this stored energy can be expressed mathematically as follows:
Pr = P + u p c
Where P is initial pressure, u and p are initial fluid velocity and density respectively and c is the speed of sound through the fluid.
Accumulators and other damping devices are sometimes installed in an effort to deal with this problem. However, the significance of the pressure rise equation shown above is that fluid velocity is the only variable that can be altered to address the root cause.
Put simply, reducing the velocity of the fluid column that hits the solid boundary, reduces the magnitude of the subsequent pressure rise.
Returning to the traffic crash analogy – the slower the car is traveling when it hits the wall, the less damage is caused.
In hydraulics, the easiest way to do this – on paper at least, is to increase the diameter of the pipe. This reduces fluid velocity for a given flow rate.
The other alternative is to control deceleration of the fluid column by choking valve switching time to the point where the pump’s pressure compensator and/or system relief valve reacts fast enough to reduce flow rate through the pipe and therefore velocity of the fluid.
Craig Cook
Another Reason For Noise In Your System That Can Give You Problems
There is another intermittent and problematic source of noise in hydraulic systems – decompression.
This problem arises because hydraulic oil is NOT incompressible. The ratio of a fluid’s decrease in volume as a result of increase in pressure is given by its bulk modulus of elasticity.
The bulk modulus for hydrocarbon-based hydraulic fluids is approximately 250,000 PSI, (17,240 bar) which results in a volume change of around 0.4% per 1,000 PSI (70 bar).
When the change in volume exceeds 10 cubic inches (160 cubic centimeters) decompression must be controlled.
The compression of hydraulic fluid results in storage of energy, similar to the potential energy stored in a compressed spring. Like a compressed spring, compressed fluid has the ability to do work.
If decompression is not controlled, the stored energy dissipates instantaneously. This sudden release of energy accelerates the fluid, which does work on anything in its path.
Uncontrolled decompression stresses hydraulic hose, pipe and fittings. It creates noise and can cause pressure transients that can damage hydraulic components.
Decompression is an inherent problem in hydraulic presses for example, due to the large volume cylinders operating at high pressures.
Although hydrocarbon-based hydraulic fluids compress 0.4% – 0.5% by volume per 1,000 PSI, in actual application it is wise to calculate compression at 1% per 1,000 PSI. This compensates for the elasticity of the cylinder and conductors and a possible increase in the volume of air entrained in the fluid.
For example, if the combined captive volume of the hydraulic cylinder and conductors on a press was 10 gallons and operating pressure was 5,000 PSI, the volume of compressed fluid would be 0.5 gallons (10 x 0.01 x 5).
This equates to potential energy of around 33,000 watt-seconds. If the release of this amount of energy is not controlled, you can expect to hear a bang!
Decompression is controlled by converting the potential energy of the compressed fluid into heat. This is achieved by metering the compressed volume of fluid across an orifice.
Craig Cook
Noisy Hydraulics
Noise level in the workplace is always an issue. The high power density and corresponding high noise emission of hydraulic components means that industrial hydraulic systems are often the target of efforts to reduce noise levels in the workplace.
The dominant source of noise in hydraulic systems is the pump. The hydraulic pump transmits structure-borne and fluid-borne noise into the system and radiates air-borne noise.
All positive-displacement hydraulic pumps have a specific number of pumping chambers, which operate in a continuous cycle of opening to be filled (inlet), closing to prevent back flow, opening to expel contents (outlet), and closing to prevent back flow.
These separate but superimposed flows result in a pulsating delivery, which causes a corresponding sequence of pressure pulsations. These pulsations create fluid-borne noise, which causes all downstream components to vibrate.
The pump also creates structure-borne noise by exciting vibration in any component with which it is mechanically linked, e.g. tank lid.
The transfer of fluid and structure induced vibration to the adjacent air mass resulting in air-borne noise.
Reducing fluid-borne noise
While fluid-borne noise attributable to pressure pulsation can be minimized through hydraulicpump design, it cannot be completely eliminated. In large hydraulic systems or noise-sensitive applications, the propagation of fluid-borne noise can be reduced by the installation of a silencer.
The simplest type of silencer used in hydraulic applications is the reflection silencer, which eliminates sound waves by superimposing a second sound wave of the same amplitude and frequency at a 180-degree phase angle to the first.
Reducing structure-borne noise
The propagation of structure-borne noise created by the vibrating mass of the power unit (thehydraulic pump and its prime mover) can be minimized through the elimination of sound bridges between the power unit and tank, and the power unit and valves.
This is normally achieved through the use of flexible connections i.e. rubber mounting blocks and flexible hoses, but in some situations it is necessary to introduce additional mass, the inertia of which reduces the transmission of vibration at bridging points.
Reducing air-borne noise
The magnitude of noise radiation from an object is proportional to its area and inversely proportional to its mass. Reducing an object’s surface area or increasing its mass can therefore reduce its noise radiation.
For example, constructing the hydraulic reservoir from thicker plate (increased mass) will reduce its noise radiation.
The magnitude of air-borne noise radiated directly from the hydraulic pump can be reduced by mounting the pump inside the tank. For full effectiveness, there must be a clearance of 0.5 meter between the pump and the sides of tank, and the mounting arrangement must incorporate decoupling between the power unit and tank to insulate against structure-borne noise.
The obvious disadvantage of mounting the hydraulic pump inside the tank is that it restricts access for maintenance and adjustment.
If hydraulic system noise remains outside the required level after all of the above noise propagation countermeasures have been exhausted, encapsulation or screening must be considered.
Craig Cook