Hydropneumatic device

North American, European, and other languages refer to hydropneumatic devices such as a hydropneumatic accumulator or pulsation damper using very different words. In many cases this can cause a great deal of word usage confusion. This article includes information about devices that prevent, but do not absorb, alleviate, arrest, attenuate, or suppress a shock that already exists (these devices prevent the creation of a shock wave at an otherwise higher level). The words frequently used for devices such as pulsation dampeners, hydropneumatic accumulators, water hammer preventers, water hammer arrestors, and others, are included.

Hydropneumatic water hammer preventers

Purpose

To provide a chamber of sufficient volume to allow an extension of time in which a given flow may be accelerated or decelerated without sudden large change in pressure. See also expansion tank. When shock waves of an incompressible fluid within a piping system exist, especially at a high velocity, there is a high chance for water hammer. To help prevent a swing check from slamming and causing water hammer, a spring-assisted non-slam check valve is installed. Rather than relying on flow or gravity to be closed, the non-slam design prevents a sudden velocity decrease and reverse flow.^[1]

Characteristic

The chamber is generally adapted to contain a separator member which prevents the escape of a pre-filled compressed inert gas.

Applications

a. Placed closely before a valve that is closed quickly. Stops water hammering.
b. Placed immediately after the discharge of a pump that is started fast into a pipe full of a long column of liquid. Reduces start up surge pressure.
c. Placed immediately after a pump, which when caused to stop suddenly, enables a vacuum to form, which pulls the flow back towards the pump. Prevents an implosion bang.

Variations

a. Having a separator membrane into the interior of which the liquid is communicated. Used for corrosive liquids, so that the chamber metal can be of low cost.
b. Having a metal bellows separator membrane for use at low and higher temperatures than are compatible with an elastomeric or plastomeric membrane.
c. Having a float separator to reduce the rate of gas absorption at the liquid interface, typically used in vessel chambers larger than 500 gallons.

Hydropneumatic Pump Controllers

Purpose

a. Means of control for multiple fixed delivery volume, low cost low complexity pumps; to provide variable flow as required by small (say +or - 10 psi) pressure increase or decrease of a system.
b. Means of control for pump unloading / recirculation against no pressure, without electric pressure switches.

Characteristics

Pressure cylinders containing a movable separator member between a gas and a liquid, said moveable member causing the actuation of directional control valve or valves.

Applications

In circuit after a pump / or pumps, that is / are followed by valved side branch, and beyond a check valve / or valves, so that this device can only discharge liquid volume by a pressure fall of the system. Obviating use of pump drive energy unless a small pressure fall signals a system requirement for flow.

Variations

a. Having a protruding drive rod, cams from which trip valve handles.
b. Having magnetically actuated reed switched.
c. Having infra red signaling of separator position.

Hydropneumatic Pulsation Filters

Purpose

To provide means of reducing the amplitude of pressure changes the velocity of which is in the order of 1.4 km/s. All are used in industry.

Characteristics

Pressure container with separate inlet and outlet, connectable to a pipe system so that all pressure changes must attempt to pass through said chamber. Entry and exit of said chamber being of a diameter relative to chamber diameter that provides a high discharge coefficient, and without close proximity of any reflective surface. Lack of any sudden change in cross section area of flow path that would reflect a pressure wave, i.e. no orifice plate(s).

Applications by frequency response

For pulsation above frequency 100 Hz (i.e. for high speed pumps and all pipe systems shorter than say 80 yards): no moving parts devices. For pulsation frequencies below 100 Hz: certain moving parts devices of known membrane response characteristics.

Variations

Combination “dual purpose” devices addressing “acceleration head reduction” see 4. below by means of a gas containment plus characteristics 3.2 above.

Hydropneumatic Acceleration Head Reducers

Purpose

To minimize the mass of liquid that has to be accelerated when flow velocity changes.Within a piping system, pressure rises when a volume of fluid becomes present. This acceleration head needs to be reduced to prevent damage to pump components and excessive noise.^[2]

Characteristics

Mountable in any orientation such that the device is connectable directly to the suction check valve beneath the pump or directly to any vertical or horizontal discharge check valve; minimizing the length of any liquid column mass that will experience velocity change. Pump connection being separate from system connection so that no acceleration head changes occur due to reciprocation within one port.

Applications

a. Reduction in drive energy costs required by any pump.
b. Reduction in pipe diameter and schedule (wall thickness) costs of any pipe system.
c. Decrease in fatigue and increase in safety of all pressure piping systems.
d. Increase in accuracy and automatability of all pressure and flow control instruments.
e. Increase in rotating equipment life and MTBF.
f. Reduction in service down time.

Variations

a. For chemicals and process pump systems: having PTFE membranes.
b. For sludges and slurries: having a clear unobstructed flow path direct from in to out.
c. For general purposes: having an elastomeric bladder separator.

Hydropneumatic word usage, definition, and the internet search

The Need for Definition

North American, European, and other languages refer to hydropneumatic devices such as a hydropneumatic accumulator or pulsation dampener using very different words. Lack of definition and standardization has caused confusion, and word inference has caused a belief in the opposite of the truth. This section is for definition of the devices that prevent, and do not either absorb, alleviate, arrest, attenuate, nor suppress a shock that exists (devices which prevent the creation of a shock wave at an otherwise higher level). The words frequently used for devices such as pulsation dampeners, hydropneumatic accumulators, water hammer preventers, water hammer arrestors, and others, are included.

Pulsation Dampers - Geographical and other differences

In the United States of America

The phrase "pulsation dampeners" is used for items that are said to "dampen pulsation".The use of the plural is a North American habit.

In other parts of the Americas

The singular version, hence: I need a "pulsation dampener", is used for searching when thinking in American English. In South America the singular is used "amortiguador de pulsaciones"; but the pulsaciones is plural because pulsations are, by definition, multiple occurrences.

In Europe/United Kingdom

In Europe the English speakers in the UK follow the American habit of searching using the plural version, but they frequently shorten the search to "pulsation dampers"., or even “pulse dampers”.

In other parts of Europe

The rest of the English language users tend to use the singular, e.g. "I need a "pulsation damper." Similarly, in Germany, they habitually look for one of an item; but the phrase "pulsationsdämpfer" happens to mean both singular and plural. "Pulsations" plural, is used in both Nordic and Latinate languages (except UK English) because pulsation is by definition a repetitive occurrence. Example: the French "amortisseur de pulsations" is plural. However the devices are typified by having only one liquid connection that goes to a "T" on the system.

Word Misuse

Approximately 120 manufacturers of pulsation dampers provide items which do not damp. The authority for saying this is, because the words dampen and dampener have at their root, removing energy.
How the word misuse has arisen
The compressibility of a gas, often nitrogen because it is inert at normal temperatures, stores any sudden volume change. Storing sudden volume change enables volume to change against a soft gas cushion, without the need to accelerate all the existing liquid in the system out of the way of the new volume coming from a pump. Therefore, as all the volume in a system does not have to be suddenly accelerated, the cushion is preventing "acceleration head (force)" having to be generated. The pressure pulse is accordingly not generated in the first place, so it is not damped at all. The gas cushion simply allows volume change to be stored. The manufacturers are providing, are liquid accumulators, not an item which removes energy.

Hydropneumatic Accumulators

Why "Hydropneumatic"

Gas cushion (spring) pre filled, accumulators of liquids, are defined as: hydropneumatic accumulators. "Hydro" because a liquid (like water) is involved. "Pneumatic" because a gas (like air) is involved.

Why "Accumulator"

"Accumulator" because the purpose is to store or accumulate liquid volume by easy compression of the gas. These devices are typified by having only one liquid connection that goes to a "T" on the system.

Non "Hydropneumatic"

There are other forms of accumulator used for fluid power hydraulic purposes. For example, coil spring plus sealed piston; though these are less popular. Therefore, a hydraulic accumulator is not necessarily a hydropneumatic accumulator.
American English
In American English language, the search is for hydraulic accumulators.
American English outside the US
In American English outside the USA, the search is for hydraulic accumulator, the singular.
UK English
In UK English the search in the UK, is for hydropneumatic accumulators, plural.
UK English outside UK
In UK English the search outside the UK, is for hydropneumatic accumulator, singular.
Germany
In Germany the search is for Druckspeicher (pressure store), singular and plural; and some times hydropneumatischer Akkumulator.
French
In French the search is for accumulateur hydropneumatique, singular.
Spanish
In Spanish the search is for accumulator hidrapneumatic
In Dutch the search is for waterslagdemper, singular

Liquid Shocks, Surge Pressures, and Water Hammer Prevention

Confusion

There is a confusion from the large numbers of phrases in the activity of preventing liquid shocks, surge pressures, and "water hammer". There is general confusion because there are at least sixty English two or three word phrases with the same general meaning. The following need to be used in both singular and pluralization for internet searches:

Shock: Shock absorber, shock alleviator, shock arrestor, shock attenuator, shock suppressor, shock preventer.
Surge: surge absorber, surge alleviator, surge arrestor, surge attenuator, surge preventer.
Water Hammer: water hammer absorber, water hammer alleviator, water hammer arrester, water hammer attenuator, water hammer suppressor, water hammer preventer.Confusion from word implications

The wrong implications

Most of these phrases imply that the shock, the surge, or the water hammer is addressed after it has been created.In fact most of those phrased apply to hydropneumatic devices that PREVENT a high level of shock, surge, or hammer, from occurring at all. The named devices are rarely designed to respond to a pressure incident in a liquid which must have an approximate typical velocity of 1,440 meters per second.

Pulsation Filters, and Shock Filters

(Search words used because the words beginning with "damp-" are abused)
True Dampers
These true dampers are available from several sources, the purpose of their design is to address the typical approximate acoustic pressure wave velocities in liquids of 3500 mph. The common characteristic is interception by flow-through. They are multi connection devices that intercept and dissipate the over pressure energy of shocks, surges, water hammer, and pressure pulsation. These were called "pressure pulsation dampers and shock wave dampers". They are now "Pulse Filters", "Pulsation Filters", "Acoustic Filters for liquids".

References