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Pressure transmitter designLiquid (water) hammer phenomenonHow to protect your system from damage caused by water hammering

MAKING MODERN LIVING POSSIBLE

What is not widely acknowledged is that the situation before the pressure peak is the actual reason for the possi-ble damage of the components which form part of the system - and that this

Liquid (water) hammer also applies to pressure transmitters.If the pressure is measured before and after a valve in a system with a given liquid flow, sudden closing of the valve will cause pulsating pressure on the

Fig. 1. Test setup.The test setup comprises a liquid con-tainer, a pump, a 25 m pipeline to the valve, a pressure transmitter in front of the valve (B), a pressure transmitter be-hind the valve (C) and a 25 m pipeline from the valve back to the container. Please note that this setup corresponds to the setup illustrated in fig. 5 - 8.

inlet as well as the outlet of the valve.This can be demonstrated, both by simulation as well as in practice, on a system as outlined on Fig. 1.

Valve

Liquidcontainer

Pump

Fig. 3. Measured and simulated pressure transients behind the valve when closing

Pos. C

Fig. 2. Measured and simulated pressure transients in front of the valve when closing

Pos. B

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Pressure transientsFig. 2 and Fig. 3 show the measured and simulated pressure transients in front of and behind the valve immedi-ately after closing of the valve. The pressure transients or the variation of pressure in front of the valve (Fig. 2) correspond to what could be expected

Pressure peak is a well-known phenomenon in many systems with sudden changes in the liquid flow, for example, by closing of a valve or by stopping of a pump.

by closing of the valve. It appears from Fig. 3 that the pressure on the outlet of the valve will, how-ever, reach absolute zero, i.e., vacuum, immediately after closing of the valve. If this vacuum creates a formation of gas pockets in the dead volume in front of the pressure transmitter sensor

(see Fig. 4), by cavitation or in other ways, the subsequent rise in pressure will have the effect that liquid will be hammered against the sensor caus-ing possible damage to the pressure transmitter.

Cavitation: Formation of “gas pockets” when the pressure gets below the vapour pres-sure of the liquid. If these “gas pockets” are imploded (exploded) at a mate-rial surface, this surface is exposed to heavy mechanical stress (pressure) in the implosion area.

Liquid hammer: Liquid backlash. Due to the kinetic energy, a deceleration of liquid, for example by closing of a valve, will cause the liquid to collapse when the pressure gets below the vapour pressure of the liquid. The liquid flow continues until the “suction” power in the cavitation area has reached a size which turns the liquid

flow. When the liquid returns it will result in high pressure peaks that are non-uniformly spread on the surface that it hits.

The effects of pressure tran-sients The two figures 5 and 6 show a cross section of the pressure transmitter (B) mounted in front of the valve, the valve itself and a cross section of the pressure transmitter (C) mounted behind the valve. Fig 5 shows the liquid condition at the separation diaphragm of the pressure transmitters during the vacuum situa-tion at transmitter C (immediately after closing of the valve). Solving pressure peak prob-lemsFig 6 shows the liquid condition at the transmitters at the first pressure peak after closing. It appears that the separation dia-phragm is deformed on the transmit-ter (C) that is mounted after the valve.

This is due to a non-uniform pressure distribution on the diaphragm when the liquid hammering hits.It is not the pressure peak itself that causes damage to the sensor, but the condition before the peak occurs.This conclusion only applies to sensors with a very high degree of safety towards static overload pressure, as for example, the monolithic silicon pressure sensor. A sensor with a low degree of safety towards static overload can be damaged by the pressure peak itself.

The conclusion also explains why the problem can be observed at a pressure range of 1 bar as well as at a pressure range of 400 bar.In the test system, the actual prob-lem does not occur on the inlet of the

Fig. 4. Cross section of MBS pressure connection

Fig. 5. Vacuum and cavitation occurs at the sensor and in the pipe after closing the valve due to liquid kinetic energy.

Danfoss pressure transmitters type MBS are based on a pi-ezoresistive, monolithic silicon pressure sensor, protected by a stainless steel separation diaphragm and silicone oil filling between the diaphragm and the sensor (fig. 4).

valve, but, on the con-trary, it oc-curs on the out let (Fig. 3). In practice, it is therefore often difficult to find the right place to look for the problem. The phenomenon has been observed on various applications, thus it is now possible to point them out in advance and propose a solution. The solution consists of a specially designed nozzle with a precision laser drilled orifice mounted in the pressure port of the transmitter. The function of the nozzle is not to protect against pressure peaks. To a very high degree this is secured by the nature of the silicon pressure sensor (overload pressure: 10 - 20 × measuring range). The function of the nozzle is to pre-vent the gas pockets that may result in the creation of the dangerous non-uniform pressure peaks (cavitation and liquid hammer) shown in Fig. 5 and Fig. 6. A benefit of the nozzle design solution is that it does not increase the total time constant perceptibly:Mechanical + electronic time constant is typically 1 mS for Danfoss pressure transmitters.For viscosity larger than standard hydraulic oil (32 cSt), it can be stated as a rule-of-thumb that the time constant in mS increases by approximately 2 - 3% of the viscosity increase in cSt (mm2/s).

Fig. 6. Liquid backlash can create high pressure peaks of a non-uniform nature at the sensor diaphragm that may destroy the diaphragm.

Design of Pressure Transmitters type MBSSeparation diaphragm

Piezoresistive sensor Oil filling

B C

B C

Valve

Valve

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Conclusion All pressure transmitters in the HEAVY DUTY series based on the piezo resis-tive design are delivered with the specially designed nozzle and thus comply with our philosophy that pressure must not be a problem for a pressure transmitter (gauge) regardless of how erratic the pressure appears, as long as the stated maximum limits for the pressure range in question have been complied with.

Fig. 8Liquid backlash creates high pressure peaks. Due to the uniform pressure profile at the diaphragm and the high degree of safety of the silicon chip to-wards overpressure, the pressure peak will cause no damage of the sensor.

Fig. 7After closing the valve vacuum and cavitation only occurs in the pipe due to the restriction in the nozzle.

The problem does of course not occur in all liquid systems, but our experi-ence show that the phenomenon can occur in a wide range of plant, such as hydraulic presses, mobile cranes, braking systems for windmills, diesel engines, refrigeration systems with NH3, etc. Prevention Prevention of “the liquid hammer phenomenon” must take place during dimensioning of the plant.

In situations where it is not possible to eliminate liquid hammer problems by dimensioning it is necessary to use components that resist the operating and environmental influences they are exposed to.By using Danfoss pressure transmitters of the HEAVY DUTY series a reliable pressure measurement is obtained, also during critical conditions.

IC.PB.P20.L1.02 / 520B5038 Danfoss A/S, April 2012 /hat

Danfoss A/S, Industrial AutomationDK-6430 Nordborg · Denmark · Tlf: +45 7488 2222 · ia@danfoss.com · www.danfoss.com/ia

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B C

B C

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