Rubber Lining Application Manual
Revision Date: September 1, 2009
Section 11: Curing
Page: 11 - 1
CURING
The five basic methods used for curing tank lining are autoclave, internal (pressure) steam,
exhaust or atmospheric steam, chemical cure, and hot water cure. The specific method
used will depend on the nature and size of the vessel to be lined. Note that
recommendations contained in this manual are suggested guidelines only. Actual cure
times will be dependant on factors such as rubber thickness, vessel size and wall thickness,
heat loss, ambient conditions and elevation.
Your Polycorp representative should be consulted concerning major changes in cure. All
cures should have proper temperature recording charts and these should be properly
identified with job number and date. If there are any questions regarding conformance of
cure to published time and temperature recommendations, please contact your Polycorp
representative.
Rubber Lining Application Manual
Section 11: Curing
Revision Date: September 1, 2009
Page: 11 - 2
1. AUTOCLAVE CURE
This refers to vulcanization where the rubber-lined vessel is placed inside a pressure vessel
and subjected to controlled steam under pressure. An autoclave cure provides the best
and most uniform cure and should be used whenever possible. Metal parts should be
placed in the autoclave so that the best possible drainage of condensate from the rubber will
be obtained. To obtain the most accurate and uniform cure, it is desirable to have the
autoclave fully equipped with thermocouples and instrument controls on air pressure and
steam. The incidence of improper cure is more likely to occur when using a manually
operated method. Sufficient boiler capacity should be available to raise the temperature
from ambient to cure in a relatively short period of time. At termination of cure, it is
recommended the rubber-lined vessel be cooled down by using water and/or air. Proper
cool down of autoclaves will prevent post curing and preclude the possibility of cracking hard
rubbers. The following cool-down procedures are suggested as recommended methods:
a) Cool down soft natural and synthetic rubbers one hour with air and water.
b) Cool down Triflex and hard rubber with air and water until autoclave temperature
reaches 200°F. Continue a gradual cooling down of the autoclave with air and then
with air and water. This cool down procedure can be modified, but a step-wise
procedure will prevent cracking of the hard rubber.
During cool down it is important to maintain an air and water pressure equal to or greater than
the steam pressure.
All autoclaves should be equipped with temperature and pressure recorders. The recording
charts should be properly identified and dated. An autoclave is shown in sketch 1401 with
details of operation on following pages.
Precautions must be taken against stratification of steam and air particularly in large
vulcanizers. During start-up the bottom exit valve must be cracked open to allow a complete
sweep of steam and cold air through the autoclave to avoid a cold bottom and subsequent
undercured rubber.
Rubber Lining Application Manual
Section 11: Curing
Revision Date: September 1, 2009
Page: 11 - 3
Sketch 1401 TYPICAL AUTOCLAVE – POLYCORP PROTECTIVE LININGS
AUTOCLAVE OPERATION (BASED ON THE POLYCORP DESIGN ABOVE)
1.
2.
3.
4.
5.
6.
7.
8.
9.
Close drain valve.
Close blow off line.
Close air line valve.
Close cooling water valve.
Open trap line valve.
Set temperature desired on automatic control.
Open vent line valve.
Open steam valve.
Automatic control valve on vent should close when autoclave reaches desired
temperature.
10. Proceed with specified cure.
11. At end of cure, open-air line valve, close steam valve, open cooling water valve
and close vent line valve.
Rubber Lining Application Manual
Section 11: Curing
Revision Date: September 1, 2009
Page: 11 - 4
12. Maintain an air and water pressure equal to or greater than the curing pressure.
The cooling water pressure must exceed the holding air pressure used for
cooling. This is necessary so water will enter the autoclave.
13. Cool down soft rubber one hour with air and water.
14. Cool down Triflex and hard rubber lined tanks with air and water until autoclave
temperature reaches 200°F; then with air for one hour; then with air and water for
one hour. This gradual cool down procedure precludes the possibility of cracking
the hard rubber lining.
15. Turn off air.
16. Turn off water.
17. Open blow off valve.
18. Open drain valve.
19. When pressure is reduced to zero, open autoclave.
Rubber Lining Application Manual
Section 11: Curing
Revision Date: September 1, 2009
Page: 11 - 5
2. INTERNAL OR PRESSURE STEAM CURE
Internal steam pressure cures are used on vessels that are designed for pressure and are
too large to be placed in an autoclave. The vessel should be positioned during cure so that
complete condensate drainage is obtained. To accomplish this, tanks with a sump should
be cured with a well pipe connected to a trap (see sketch 1402). Tanks without a sump can
be cured with a steam trap and temperature recorder connected to a bottom outlet (see
sketch 1403). All nipples should be bled with a 1/4” petcock. Bottom outlets should be left
open long enough to be sure that all air is evacuated before building up pressure.
Sufficient boiler capacity should be available to raise the temperature from ambient to curing
in a relatively short period of time. Long uninsulated pipe runs from the steam source
should be avoided. Low pressure steam plus uninsulated lines promote excessive
condensate.
A thermometer and pressure gauge should be placed in a blind flange near the bottom of
the tank and when the thermometer registers the temperature corresponding to the gauge
pressure, the air has been fully evacuated.
Prior to introducing steam into the vessel all outlets should be blanked off. Flanges should
be lined with one of the special constructions shown in sketches 1404, 1405, or 1406. This
flange procedure will allow the vessel to be bolted up under pressure without undue
distortion of rubber on face of flange. Contact your Polycorp representative for further
information.
Cool down internal steam cures by introducing air until cold. Some natural rubber
compounds tend to develop tacky surfaces when air is introduced too quickly into the
vessel. If this occurs, the curing pressure should be allowed to drop 5 to 10 psi before air is
introduced. The air pressure must be maintained at five pounds higher than the curing
pressure. All cures should have proper temperature and pressure recording charts and
these should be properly identified with job number and date.
Outside temperature has a significant influence on the time required to cure a vessel. In
cold winter temperatures, it is imperative that the vessel be insulated to effectively carry out
the cure. A cold winter factor cannot be offset by internal steam temperature.
Steel thickness is also a factor. Heavy thick steel needs additional time to compensate for
the heat sink and warm-up period. This is where external temperature gauges are quite
valuable in monitoring the time/temperature so one can judge and insure themselves that a
positive cure is being obtained.
Rubber Lining Application Manual
Revision Date: September 1, 2009
Section 11: Curing
Page: 11 - 6
Sketch 1402 – Internal Steam Cure – Tank with Sump
Rubber Lining Application Manual
Revision Date: September 1, 2009
Section 11: Curing
Sketch 1403 – Internal Steam Cure – Tank with Bottom Outlet (No Sump)
Page: 11 - 7
Rubber Lining Application Manual
Revision Date: September 1, 2009
Section 11: Curing
Page: 11 - 8
Sketch 1404 – Internal Steam Cure – Flange Detail for Outlets
(Recommended for Vacuum Equipment)
Sketch 1405 – Internal Steam Cure – Flange Detail for Outlets
Rubber Lining Application Manual
Revision Date: September 1, 2009
Section 11: Curing
Sketch 1406 – Internal Steam Cure – Flange Detail for Outlets
(NOT Recommended for Vacuum Equipment)
Page: 11 - 9
Rubber Lining Application Manual
Section 11: Curing
Revision Date: September 1, 2009
Page: 11 - 10
3. EXHAUST OR ATMOSPHERIC STEAM CURE
Exhaust or atmospheric steam is normally used for field vessels that have open tops and/or
bottoms, vessels that do not withstand pressure or vessels that are too large to fit in an
autoclave. In the case of open top tanks, the opening must be covered with fabric or
combinations of fabric and plastic. The covering must be made tight so the steam will be
contained. Steam should be introduced into tank by means of a steam line through an outlet
or the opening in the top of tank. It is absolutely essential that provisions be made to drain
all condensate from the vessel during cure. The bottom outlet on a closed top tank may be
left open for drainage, but other outlets should be closed with blind flanges drawn up to
within 1/32” of rubber on flange.
During the first stages of cure, steam should be introduced for approximately 1 hour. This
time may vary according to the size of tank or size of steam line. The time should be long
enough to expand any trapped air so that it may be found and repaired, and short enough
so that the surface of the stock will not be cured to the stage where a needle patch repair
cannot be made.
After this initial warm up period, the steam could be turned off and the lining inspected while
hot. At this stage, any visible trapped air blisters can be punctured with a hypodermic
needle to remove the air. After air has been removed, the needle puncture should be
repaired with a “needle patch” of the uncured rubber. Needle patches should be firmly
stitched down to close the needle hole in the underlying stock.
On completion of the needle patch repair, the steam should be again turned on in sufficient
force to bring the temperature higher than 200°F. The volume of steam should be enough
so that some live steam can be seen blowing out the bottom outlet. The steam should not
impinge directly on the rubber lining.
Tanks that have an enclosed top with bottom outlet should start timing the cure when hot
steam at 200°F minimum is coming out the bottom outlet and the temperature of the rubber
surface has reached the desired cure temperature (as measured in the coldest spot in
the vessel). The length of cure is counted from the time the temperature reaches the
requirements until the end of cure when the steam is turned off.
Thermometers through outlets and internally located thermocouples at various points in the
tank should be used to monitor the temperature. When temperature varies at different spots
in the vessel, the coldest spot should govern how much the overall cure should be
increased. No cool down period is required for this type of cure. If steam at atmospheric
pressure cannot be obtained, the cure will have to be adjusted.
Rubber Lining Application Manual
Section 11: Curing
Revision Date: September 1, 2009
Page: 11 - 11
EXHAUST STEAM CURING PRECAUTIONS:
Specifications and information on the technical data sheets is written based on consistent
lab conditions. Frequently the cure times are extended for the lining to receive the optimum
cure. The following are all considerations in determining the correct cure cycle.
•
Warm up vessel at a rate of 10-15 degrees/hour. The time it takes to warm up the
unit does not count toward the cure time.
•
Curing is a function of time and temperature. Lower temperatures will lengthen the
cure time. Tank insulation, temperature of the surrounding air, elevation and wind
velocity over the tank can shorten or lengthen the cure.
•
Thickness and surface area (structure) of the metal will act as a heat sink and
increase the length of the cure. The metal structure must reach a constant
temperature before the cure time is started.
•
Cold spots can develop as a result of trapped air, condensate accumulation or
ineffective steam circulation. Thermocouples must be strategically placed to
monitor the coldest spots in the tank. Temperature readings must also be taken on
the metal opposite the rubber lining to ensure the metal temperature is high enough
to cause the rubber/cement interface to cure.
•
The amount of steam must be properly estimated for curing. To ensure proper cure
of flange faces, steam must escape around openings. If the steam is stagnant, cold
spots form. Petcocks may be installed on extended piping and shall be used to
insure steam flows through those areas and across the flange faces. The steam
supply must not only be adequate but must be directed in such a way to ensure it is
flowing over all rubber lining surfaces, especially the bottom. All condensation must
be removed. Drains should be provided at the bottom of the vessel and elsewhere
to ensure condensate will run off and not collect in any pockets. An open top tank
or one sitting on a concrete pad can result in situations where the tank walls are
cured but the floor is not. The steam must be directed towards the bottom of the
tank and flow around all surfaces.
•
The entire cure and cool down must be monitored, especially if it is possible to pull a
vacuum and collapse the vessel. Vacuum breakers may be necessary depending
on the construction of the vessel.
Rubber Lining Application Manual
Revision Date: September 1, 2009
Section 11: Curing
Page: 11 - 12
TYPICAL EXHAUST STEAM PIPING SETUP
Note:
For large tanks or during cold weather, larger size piping to furnish more steam
may be required (insulation may also be necessary) to maintain appropriate
inside curing temperature that is in accordance with Polycorp’s recommendation.
Rubber Lining Application Manual
Section 11: Curing
SAMPLE CURVES FOR ATMOSPHERIC CURING
Revision Date: September 1, 2009
Page: 11 - 13
Rubber Lining Application Manual
Revision Date: September 1, 2009
Section 11: Curing
Page: 11 - 14
KEY CALCULATIONS FOR STEAM REQUIREMENTS FOR ATMOSPHERIC CURES
a) Pounds of Steam per hour: Example - during 2-hour warm-up period and based on an
enclosed vessel with 3/8” plate on bottom and 1/4" on sides and top;
Pounds per hour of steam = 0.05V + 0.15A + 0.005W
Where:
V = Volume in cubic feet
A = Area in square feet
W = Weight of rubber and steel in pounds
b) Boiler Horsepower Required = Pounds of steam per hour ÷ 30
c) Steam Flow Table: Table shows steam flow in pounds per minute through 1,000 feet of
pipe
Steam Pressure
Pipe Size
50#
75#
100#
1”
2”
2-1/2”
3”
3-1/2”
4”
4.27
25.8
40.7
71.5
89.0
144.0
6.32
36.7
57.7
101.0
146.5
204.0
8.15
52.5
74.5
130.0
185.0
263.0
To the use the table, determine steam requirement in pounds per minute for 1,000 feet
of pipe. For example, if 2,0000 pound per hour are required (2000/60 = 33.3 lb./minute),
a 2” pipe at 75 pounds of steam pressure is required.
To adjust for varying lengths of pipe, multiply flow rate from the table by the appropriate
factor below:
Length of Pipe (feet)
Factor
100
200
300
400
500
600
700
800
900
3.16
2.24
1.83
1.58
1.41
1.29
1.20
1.12
1.05
Example: If steam source is 100’ away, a 2” pipe is used and gauge pressure is 50#
then steam delivery/minute = 3.16 x 25.8 = 81.53 pounds.
Rubber Lining Application Manual
Section 11: Curing
Revision Date: September 1, 2009
Page: 11 - 15
4. CHEMICAL CURING
There are cases where using heat to cure the rubber is not possible. Steam may not be
available, or the equipment to produce the steam is inadequate. Chemical curing rubber is
an acceptable solution for these situations. It is typically used in field repair situations.
Chemical activators are used to cure these specialized compounds. They are almost
always used by coating the activator on the rubber lining sheet and then adding another
coat on the lining after the first has dried. In some cases you may want to coat the rubber
sheet prior to lay-up. Specific instructions are contained on the respective Technical Data
Sheets in this manual. Activator should not be applied to the side of the rubber being
applied to the substrate as this may interfere with bonding.
The chemical activator will only result in a rapid cure of the lining surface. Full cure of these
types of linings may take weeks. The time required is dependent upon the type of activator
and temperature. Steam, heat lamps or other heat sources may be used to accelerate the
curing process. Chemical cure linings may also be restricted to thinner gauges as the
activator my not penetrate the full thickness of heavier gauge materials.
Care must be taken to ensure no dynamic forces (chemical or physical) will be applied to the
lining until it has cured throughout.
Rubber Lining Application Manual
Section 11: Curing
Revision Date: September 1, 2009
Page: 11 - 16
5. HOT WATER CURING
On field lined tanks, one way to obtain some pressure during cure is to use water for
hydrostatic pressure. In preparation for water cure, the outlets and/or manways must be
prepared using chemical cured or pre-cured rubber. Alternatively, the outlet may be exhaust
cured so the manway covers and blind flange plates can be tightened to hold the water.
Cured rubber gaskets are also required. The other area requiring preparation is to extend a
stand pipe column (pipe) for ten feet above the vessel structure with a hose to exhaust air
and/or steam vapors during the curing process. The tank structure must be designed so
that it is able to withstand the hydrostatic pressure of the water and curing temperatures up
to 210ºF. Also, the tanks need to have sufficient foundation to support the water filled
vessel, either a sand base for non-installed, or the proper tank setting itself.
On open top tanks, the preparation procedures are the same, except the tank is lined all the
way to the top and the water is filled to the top.
Prior to filling the tank with water, steam sparger piping should be installed near the bottom
of the vessel. The steam itself shall be directed not to impinge on the rubber surface and it
is recommended that the steam be introduced in an angled, downward direction. Steam
should be introduced to the filling tank as soon as the lowest sprayer nozzles are immersed.
Once the temperature has reached 180ºF, the cure can be considered as starting. It is best
for the water temperature to be 180ºF – 205ºF. No water source is acceptable that may
contain oil.
Thermocouples should be used to monitor the temperature within the vessel from top to
bottom. Also, metal temperature probes may be utilized on the outside for temperature
information if the tank is not insulated.
After the cure is completed, it is recommended that the water be allowed to cool down to
approximately 110ºF before draining the vessel. This reduces the likelihood of trapped air
expansion blisters. If the water temperature is slow to come down, it is permissible to add
cold water.
Rubber Lining Application Manual
Revision Date: September 1, 2009
Section 11: Curing
Page: 11 - 17
PROPERTIES OF SATURATED STEAM AT SEA LEVEL
GAUGE
PRESSURE
(in psi)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
62
64
66
68
70
ABSOLUTE
PRESSURE
(psia)
14.7
15.7
16.7
17.7
18.7
19.7
20.7
21.7
22.7
23.7
24.7
25.7
26.7
27.7
28.7
29.7
30.7
31.7
32.7
33.7
34.7
36.7
38.7
40.7
42.7
44.7
46.7
48.7
50.7
52.7
54.7
56.7
58.7
60.7
62.7
64.7
66.7
68.7
70.7
72.7
74.7
76.7
78.7
80.7
82.7
84.7
TEMPERATURE
ºF
212.0
215.4
218.5
221.5
224.5
227.4
230.0
232.4
234.8
237.1
239.4
241.6
243.7
245.8
247.9
249.8
251.7
253.6
255.4
257.2
258.8
262.3
265.3
268.3
271.4
274.0
276.7
279.4
281.9
284.4
286.7
289.0
291.3
293.5
295.6
297.7
299.7
310.7
303.6
305.5
307.4
309.2
310.9
312.7
314.3
316.0
SENSIBLE
HEAT
(Btu/lb)
180.2
183.6
168.8
189.8
192.7
195.5
198.1
200.6
203.1
205.5
207.9
210.1
212.3
214.4
216.4
218.4
220.3
222.2
224.0
225.8
227.5
230.9
234.2
237.3
240.2
243.0
245.9
248.5
251.1
253.7
256.1
258.5
260.8
263.0
265.2
267.4
269.4
271.5
273.5
275.3
277.1
279.0
280.9
282.8
284.5
286.2
LATENT
HEAT
(Btu/lb)
970.6
968.4
966.4
964.5
962.6
960.8
959.2
957.6
956.0
954.5
952.9
951.5
950.1
948.6
947.3
946.0
944.8
943.5
942.4
941.2
940.1
937.8
935.8
933.5
931.6
929.7
927.6
925.8
924.0
922.1
920.4
918.6
917.0
915.4
913.8
912.2
910.7
909.2
907.8
906.5
905.3
904.0
902.6
901.2
900.0
898.8
TOTAL
HEAT
(Btu/lb)
1150.8
1152.0
1153.2
1154.3
1155.3
1156.3
1157.3
1158.2
1159.1
1160.0
1160.8
1161.6
1162.3
1163.0
1163.7
1164.4
1165.1
1165.7
1166.4
1167.0
1167.6
1168.7
1170.0
1170.8
1171.8
1172.7
1173.5
1174.3
1175.1
1175.8
1176.5
1177.1
1177.8
1178.4
1179.0
1179.6
1180.1
1180.7
1181.3
1181.8
1182.4
1183.0
1183.5
1184.0
1184.5
1185.0
SPECIFIC
VOLUME
STEAM
(cu ft/lb)
26.80
25.20
23.80
22.50
21.40
20.40
19.40
18.60
17.90
17.20
16.50
15.90
15.30
14.80
14.30
13.90
13.40
13.00
12.70
12.30
12.00
11.40
10.80
10.30
9.87
9.46
9.08
8.73
8.40
8.11
7.83
7.57
7.33
7.10
6.89
6.68
6.50
6.32
6.16
6.00
5.84
5.70
5.56
5.43
5.31
5.19