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Refrigeration

Liquid Hammer

Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 1

Refrigeration is all about phase change

- So is Liquid Hammer


Refrigeration

Liquid Hammer


Estimation of pressure spikes:

P = (mass) x (original velocity-0)

Time needed to bring liquid to rest = M x V

T

Pressure spike

Pressure spike

T 0,02 sec approx. closing time for solenoid valve

Condensing (collapsing) vapour

P = M x V T T 0 => P

1

2

Pressure

spike 1

Pressure

spike 2

Pressure spike 2

High pressure

Subcooled liquid Cold vapour

Opening and Closing Valves in Liquid Lines

It is more

important to open

valves slowly

rather than close

slowly


Refrigeration

Liquid Hammer


Collapse of Vapour in Liquid Lines

SVA stop valve exposed

to frequent liquid hammer

caused by collapsing

vapour in liquid line

Pressure spike Pressure spike

Pressure spike

High pressure

Subcooled liquid Cold vapour


Refrigeration

Liquid Hammer


Hot Gas Defrost on All Systems

• Keep hot gas lines drained for liquid

• Avoid liquid traps

• Consider using slow opening hot gas valves (MRV motor valve)

• Consider using back pressure regulators on hot gas lines

Recommended guidelines:

Hot gas Hot gas

Hot gas

Hot gas

Hot gas

To ”wet” suction line

Liquid

Max. 6 bar

To evaporator

PM

CVC EVM CVC back pressure regulator

10 bar

M

MRV

Motor valve


Refrigeration

Liquid Hammer


Collapse of Vapour in Liquid Lines

Previous FA filter insert exposed to liquid hammer

caused by collapsing vapour in liquid line

New version of FA filter inserts (different size FA

filter) with improved strength against liquid hammer

A

B


Refrigeration

Liquid Hammer


Condensed Liquid in Hot Gas Lines

Internal parts from solenoid valve

exposed to liquid hammer from

condensed liquid in hot gas lines.

Typical damage

Hot gas Hot gas Hot gas

Liquid


Refrigeration

Liquid Hammer


Solenoid valve in hot gas line

exposed to liquid hammer from

condensed liquid in hot gas line

Dangerous damage


Hot gas Hot gas

Liquid


Refrigeration

Liquid Hammer


EVM pilot valve exposed to condensed liquid in

hot gas line. Pressure ”spike” Ps > 220 bar

New improved EVM can handle pressure ”spikes”

of 600 bar

A

B

A

B


Hot gas Hot gas

Liquid


Refrigeration

Liquid Hammer


Liquid hammer in hot gas line has caused serious

deformation of the teflon seat under the piston

The valve was returned because of leaking problems with

the EVM hot gas pilot

Liquid Hammer in Hot Gas Lines

Note!

Deformation


Refrigeration

Liquid Hammer


Never inject hot gas under liquid

• Vapour in front of liquid may collapse and vapour propelled liquid might

”hammer” into valves, bends etc. and cause serious damage.


”Wet” suction

Liquid supply

Hot gas

Hot Gas Defrost Liquid Overfeed Systems

Applicatio

n example

Hotgas under

liquid


Refrigeration

Liquid Hammer


Hot Gas Under Liquid at Hot Gas Defrost

PM valve in suction line used as

pressure regulator and on/off

solenoid valve


Refrigeration

Liquid Hammer



PM valve used as defrost pressure

regulator Pressure ”spike” (Ps) at

liquid hammer: Ps > 500 bar

Hotgas under

liquid


Refrigeration

Liquid Hammer



PM valve in drain line

used as pressure

regulator during defrost

Hotgas under

liquid


Refrigeration

Liquid Hammer


Glass Evaporator at Danfoss Customer Center

Illustrates liquid and vapour locations in an

evaporator during operation


Refrigeration

Liquid Hammer



Illustrates liquid and vapour locations in

an evaporator during operation


Refrigeration

Liquid Hammer



Illustrates liquid and vapour locations

in an evaporator during operation


Refrigeration

Liquid Hammer


Flooded Evaporators Hot Gas Defrost

Collapsing vapour

Suction

Liquid

Impact

Hot gas

• Leaking gaskets on valves and

flange connections

• Retightening of flanges

necessary

• Possible valve damage

• Possible coil damage

Problems seen on hot gas

initiation:

Impact

Counter measures/Guidelines:

• Evacuate most liquid before

opening hot gas valve (Pump

down)

• Let liquid ”settle” in bottom of

evaporator without vapour

boubles (timer)

• Use slow opening hot gas

valves (motor valve type

MRV) or use back pressure

regulators on hot gas line

(PM+CVC)

Coil

Liquid


Refrigeration

Liquid Hammer


• Gasket from valves which have been

leaking because of liquid hammer

• The gaskets were changed several

times

• The gasket assemblies had to be

retightened several times

• On the same plant was many identical

valves and gasket assemblies in the

same application that never leaked

Liquid Hammer in Connection With Hot

Gas Defrost


Refrigeration

Liquid Hammer


Use solenoid valve in the liquid supply line

• No pump down can be performed

• Two step suction line valves might not open step two as evaporator

pressure cannot be pumped down only with suction valve step one open

• When evaporator is full of liquid, it is most vulnerable to vapour propelled

liquid


”Wet” suction

Liquid supply

Hot gas

Applicatio

n example


No solenoid

valve


Refrigeration

Liquid Hammer


Closing Times for Hot Gas Powered

Solenoid Valves

PMLX two step suction valve exposed to hot gas with

vapour propelled liquid while it is closing.

Result of opening hot gas injection before suction valve has

finished closing

High amounts of energy are lost while both hot gas line and

suction lines are open

Closing times for hot gas powered solenoid

valves depend on pressures and

temperatures.

Condensed liquid on piston needs to drain,

while evaporating

Valve Approx. Closing times in s at -40°C

R22 R717

PML(X) 32 15 45

PML(X) 40 25 75

PML(X) 50 43 129

PML(X) 65 85 255

PML(X) 80 80 240

PML(X) 100 130 390

PML(X) 125 230 690


Refrigeration

Liquid Hammer


• Inject hot gas upon liquid

• Use solenoid valve in the liquid line

• Make pump down before opening for hot gas

• Waiting time while suction valve close before open hot gas and keep fans running

• Waiting time before opening hot gas with fans stopped to make liquid settle

• Close liquid supply every time fans stops to avoid too much liquid in evaporator


”Wet” suction

Liquid supply

Hot gas

Applicatio

n example



Refrigeration

Liquid Hammer


Hot Gas Defrost Control / Sequence

Recommended guidelines !

Flooded air coolers

LIQUID SUCTION HOT GAS FAN TIMER REMARK

EVRA/T PMLX - GPS EVRA/T - PM SEC

COOLING OPEN OPEN CLOSED ON COOLING

REQUEST FOR DEFROST CLOSED OPEN CLOSED ON 120-600 EVACUATION OF LIQUID (PUMP DOWN)

TIME DEPENDING ON EVAPORATOR SIZE

CLOSED CLOSING CLOSED ON 45-700 CLOSING TIME FOR HOT GAS POWERED

SUCTION VALVE. TIME DEPENDING ON VALVE SIZE

REFRIGERANT AND EVAPORATING TEMPERATURE

CLOSED CLOSED CLOSED OFF 10-20 ALLOWING LIQUID TO SETTLE IN BOTTOM

OF EVAPORATOR WITHOUT VAPOUR BOUBLES

DEFROST CLOSED CLOSED OPEN OFF 1200-1800 DEFROST TIMER DEPENDS ON APPLICATION

STOP DEFROST CLOSED CLOSED CLOSED OFF 60-200 DRAIN TIME - PRESSURE EQUALISING

CLOSED OPEN CLOSED OFF 100-300 TWO STEP SUCTION VALVE OPENS SLOWLY

STATIC COOLING 1.(FREEZING WATER DROPS)

OPEN OPEN CLOSED OFF 60-100 STATIC COOLING 2.(FREEZING WATER DROPS)

COOLING OPEN OPEN CLOSED ON COOLING


Refrigeration

Liquid Hammer


VE

LO

CIT

Y (

v)

PR

ES

SU

RE

(P

)

Cavitation/Flashing

Important factors:

• The pressence of vapour

boubles in the liquid

before valve

• Vapour pressure:

depends on subcooling

• Pressure at vena

contracta: depends on

pressure drop over valve

• Pressure recovery: valve

design/selection and

pressure drop

P1V1 VvcPvc P2V2

P2 => Cavitation in valve P1

Pvc

V1

Vvc

V2

Vena Contracta

Vapour pressure

P2 => Flashing in valve

Creation of bubbles Collapse of bubbles


Refrigeration

Liquid Hammer


Cavitation damage on orifice in

EVM solenoid valves and TEA

expansion valve orifice

Cavitation

Cavitation damage/

wire drawing

Important factors:

• Small leaks or dirt makes

”wire drawing”

• Pressure drop over valve in

liquid line


Refrigeration

Liquid Hammer


Cavitation/Erosion

Cavitation damage: TEAT

expansion valve orifice

Important factors:

• Subcooling of the liquid (vapour pressure

at vena contracta)

• Low velocity in liquid line

• No vapour boubles in liquid line

• Stable condensing pressure, quick

variations makes vapour boubles

Liquid injection oil cooling on

screw compressor


Refrigeration

Liquid Hammer


Cavitation damage on NRVA check valves

• Small leaks creates wire drawing

• Dirt can create small leaks

Cavitation

Wire drawing


Refrigeration

Liquid Hammer


AKVA expansion valve nozzles

damaged by cavitation

Cavitation


Refrigeration

Liquid Hammer


Special ”soft” seat materials introduced

in AKVA orifices. The ”soft” seat is

special resistant against cavitation and

reduces the cavitation impact

Cavitation


Refrigeration

Liquid Hammer


Danfoss Industrial Refrigeration Research & Development

works on new valves

Cavitation and erosion in MEV test valve after 15 months of

testing on a R717 system.

The test shows clearly the need for the special soft cavitation

resistant material in the nozzle area.

The cone is also exposed to cavitation and/or erosion, but the

small damage has no effect on the regulating capability.

Cavitation

Cavitation


Refrigeration

Liquid Hammer


Cavitation Erosion

MEV Test Motor Expansion Valve with Special Plast Orifice

After test as injection valve on NH3 flooded intermediate

cooler with pumps in 1 ½ year.

No traces of cavitation or erosion in valve orifice.


Refrigeration

Liquid Hammer


Parts from PMFL which got ”stuck”

The valve could periodically not regulate modulating,

due to increased friction caused by ”fretting corrosion”

Fretting Corrosion

Fretting corrosion in an

expansion valve


Refrigeration

Liquid Hammer


Erosion damage on PM valve (pressure regulator). The

valve chosen to small leading to very high velocities.

The system contained large amount of impurities (dirt).

Liquid drops might have been present in the gas

Erosion

Correct selection of valves can be

made with the DIRcalc program


Refrigeration

Liquid Hammer


Recommended for all involved in Industrial Refrigeration

IIAR gives many very useful

guidelines in Bulletin No. 116

Make Safe Systems

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