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Expect more from u s Danfoss Industrial
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 2
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 3
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 4
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 5
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 6
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 7
Solenoid valve in hot gas line
exposed to liquid hammer from
condensed liquid in hot gas line
Dangerous damage
Condensed Liquid in Hot Gas Lines
Hot gas Hot gas
Liquid
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 8
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
Condensed Liquid in Hot Gas Lines
Hot gas Hot gas
Liquid
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 9
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 10
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.
Recommended guidelines:
”Wet” suction
Liquid supply
Hot gas
Hot Gas Defrost Liquid Overfeed Systems
Applicatio
n example
Hotgas under
liquid
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 11
Hot Gas Under Liquid at Hot Gas Defrost
PM valve in suction line used as
pressure regulator and on/off
solenoid valve
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 12
Hot Gas Under Liquid at Hot Gas Defrost
PM valve used as defrost pressure
regulator Pressure ”spike” (Ps) at
liquid hammer: Ps > 500 bar
Hotgas under
liquid
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 13
Hot Gas Under Liquid at Hot Gas Defrost
PM valve in drain line
used as pressure
regulator during defrost
Hotgas under
liquid
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 14
Glass Evaporator at Danfoss Customer Center
Illustrates liquid and vapour locations in an
evaporator during operation
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 15
Glass Evaporator at Danfoss Customer Center
Illustrates liquid and vapour locations in
an evaporator during operation
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 16
Glass Evaporator at Danfoss Customer Center
Illustrates liquid and vapour locations
in an evaporator during operation
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 17
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 18
• 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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 19
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
Recommended guidelines:
”Wet” suction
Liquid supply
Hot gas
Applicatio
n example
Hot Gas Defrost Liquid Overfeed Systems
No solenoid
valve
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 20
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 21
• 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
Recommended guidelines:
”Wet” suction
Liquid supply
Hot gas
Applicatio
n example
Hot Gas Defrost Liquid Overfeed Systems
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 22
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 23
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 24
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 25
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 26
Cavitation damage on NRVA check valves
• Small leaks creates wire drawing
• Dirt can create small leaks
Cavitation
Wire drawing
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 27
AKVA expansion valve nozzles
damaged by cavitation
Cavitation
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 28
Special ”soft” seat materials introduced
in AKVA orifices. The ”soft” seat is
special resistant against cavitation and
reduces the cavitation impact
Cavitation
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 29
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 30
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.
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 31
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 32
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
Expect more from u s Danfoss Industrial
Refrigeration
Liquid Hammer
Per Skærbæk Nielsen, Danfoss Industrial Refrigeration A/S – Version 2, Nov.. 2000 33
Recommended for all involved in Industrial Refrigeration
IIAR gives many very useful
guidelines in Bulletin No. 116
Make Safe Systems