Hydraulic seals for water and water-based fluids

Transcription

2012 Dutch Fluid Power Conference
Hydraulic seals for water and
water-based fluids
Nick A. Peppiatt BSc, PhD, CEng, MIMechE
Hallite Seals International Ltd at Hampton
United Kingdom
123
Contents
Page
Abstract
125
1. Introduction – the reciprocating seal system125
2. Use of water and water-based fluids
126
3. HFA fluids127
4. Applications of HFA fluids – longwall roof
supports127
5. Typical seals and bearings for HFA fluids131
5.1. Piston seals131
5.2. Rod seals132
5.3. Bearings132
6. Reciprocating seal testing132
7. Effects of corrosion133
8. Seal material compatibility135
9. Use of HFCs135
10. Conclusions136
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Nick A. Peppiatt
Abstract
The use of water as a hydraulic fluid is discussed and some examples of successful
applications of water-based fluids, particularly longwall mining roof supports, are
described. Examples of seal designs used in these applications are shown, and the effects
of seal material/fluid media compatibility considered.
1. Introduction – the reciprocating seal system
Keywords:
Seal system
HFA
Corrosion
Compatibility
HFC
The reciprocating seal system consists of the following elements:
1. Housings:
• Surface finishes – static and dynamic,
• Dimensions and
• Assembly details - chamfers.
2. Bearings:
• Metal or plastic.
3. Fluid:
• Compatibility and
• Lubricity.
4. Soft Parts:
• Rod seal, wiper, piston seal and static seals.
5. Operating conditions:
• Pressure,
• Temperature,
• Speed,
• Stroke and
• External environment.
Obviously many factors can affect the performance of the soft parts and clearly one of
major importance for the hydraulic sealing system is the fluid. Its influence on the sealing
system needs to be considered carefully. This paper examines the effects of the use of
water and water-based fluids on the sealing system.
Nick A. Peppiatt
125
2. Use of water and water-based fluids
The limitations of water as a hydraulic fluid include the following:
1. The temperature range is limited to +5 to +50 °C (refer to, for example, ISO 7745 [1]).
The low temperature limit is to prevent the water from freezing; the upper limit is
because of the high vapour pressure of water, which can lead to cavitation problems.
2. Water can encourage the growth of bacteria and fungi. This is an important
consideration in this health and safety conscious age, with the associated necessity
for risk assessments.
3. Plain water is a very poor lubricant.
4. Water is a major factor in causing corrosion of metal components.
Of course, all these limitations can be overcome by the use of oil-based fluids.
The earliest hydraulic systems, developed by Bramah and dating from the 1790s, used
plain water and the seals used in the cylinders were leather cups. Maximum pressures used
were around 550 bar. (McNeil [2]).
In the late 1800s hydraulic ring mains, using water, were set up in many cities, a notable
one being run by the London Hydraulic Power Company, which operated for about one
hundred years. The fluid, at a pressure of around 50 bar, operated cranes, lifts, presses and
capstans.
Burrows [3] has noted that as early as the 1860s additives to improve the performance of
water as a hydraulic fluid were being proposed. The hydraulic ring mains were eventually
superseded by electricity.
In the author’s experience, hydraulic systems using plain water are now rare. The British
Fluid Power Association (BFPA) did have a Water Hydraulics Committee, which produced
a technical guide in 1999 [4], but this committee has since been disbanded. On the other
hand, water-based hydraulic fluids are successfully and widely used in systems where fireresistance is required and examples are given later.
ISO 6743-4 [5] describes three basic water-based hydraulic fluids. These are:
1. HFA – oil in water emulsions or chemical solutions in water with typically more than
80% mass fraction of water (in most cases this is 95% or greater),
2. HFB - water in oil emulsions and
3. HFC - water polymer solutions with typically more than 35% mass fraction of water.
HFB fluids are rarely used and will not be considered further in this paper.
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Nick A. Peppiatt
3. HFA fluids
Typical HFA fluids have a water content of 95% or greater. Like water, they have a low
viscosity of around 1 cSt, but the additive, which makes the fluid alkaline with a pH value
of around 9 to 9.5, has three functions:
1. It is a biocide,
2. It prevents corrosion enabling carbon steel components to be used and
3. The soapiness of the fluid provides improved lubricity over plain water.
The low viscosity HFA fluids are of two types, HFAE, which are oil-in-water emulsions
and the later HFAS, which are chemical solutions in water. Scums can build up in the
HFAE types, which can cause clogging of valves and filters, but this does give them
generally a better lubricity than the cleaner HFAS type.
Higher viscosity HFAE fluids with a water content of greater than 80% have also been
developed.
4. Applications of HFA fluids – longwall
roof supports
Low viscosity HFA fluids have been used very successfully for many years in selfadvancing hydraulic roof supports for the longwall mining of minerals such as coal and
potash. The author has many years’ experience in developing and specifying reciprocating
seals for use in such fluids ([6], [7] and [8]). As the equipment is used underground and,
thus, hidden away, there is very little awareness of these large hydraulic machines outside
the mineral extraction industry.
Some examples of the largest and latest roof supports, or shields are shown in figures 1 and
2 on the next page. The coal face consists of a line of around 150 of these supports, which
support the roof of the mine whilst the shearer cuts the coal along a face length of around
300 m. The operation of such a modern longwall face is now highly automated.
The leg cylinders, which keep the roof of the mine from the floor whilst the coal is being
cut, can be clearly seen in figures 1 and 2. A cutaway drawing of a typical constant yield
double telescopic leg is shown in figure 3.
The outer stage bores of the leg cylinders shown in figures 1 and 2 are 480 mm and
500 mm respectively.
These main hydraulic leg cylinders are connected to a yield valve on the full bore side,
which controls the release of fluid from the leg when the pressure exerted by the roof
exceeds a pressure of around 450 bar. Pressure is trapped in the inner cylinder, so that the
inner cylinder pressure is the yield pressure multiplied by the area ratio of the cylinder
bores, which is often a factor of two.
Nick A. Peppiatt
127
Figure 1.
1750 tonne roof support
with 480mm bore leg
cylinders – note the
bronze plated rods
Figure 2.
Roof supports for 7 m
face with 500 mm bore
leg cylinders
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Nick A. Peppiatt
Figure 3.
Cutaway drawing of
typical constant yield
double telescopic (CYDT)
roof support leg cylinder
showing the seals and
bearings
The roof support also has a number of other cylinders including:
1. The advancing ram which pushes the armoured face conveyor forward and to drag the
support forward (to make the face self-advance),
2. The base lift ram to lift the front of the support while it is being advanced,
3. The stabilising ram to control the support linkage,
4. The side shield rams to close the space between supports and
5. The sprag rams or flipper rams to control the sprags or flippers at the front of the
support.
Some of these cylinders can be seen particularly in figure 2.
The maximum pump pressure operating these cylinders is around 315 bar.
In contrast, a typical UK home market leg from over thirty years ago is shown in figure 4.
This is again a double telescopic leg, but with an outer stage bore of 108 mm (4.250 in).
This leg has an extended length of just over one metre.
This increase in maximum leg diameter over the last thirty years is shown in figure 5. This
doubling of diameter has increased the load capability of roof support shields fourfold
and has enabled deeper cuts to be made in the coal seams (seven metres in the case of the
shield with the 500mm bore leg) increasing mine productivity enormously. At the current
time, with the maximum roof support width of 2 m, for transportation reasons, 500mm
bore cylinders appear close to the practical maximum limit.
Nick A. Peppiatt
129
Figure 4.
UK home market 108 mm
(4.25 in) bore leg
cylinder (from 1970s) in
convergence test rig
The use of HFA fluids enables standard carbon steels to be used in the construction of
the cylinders, although the glands often have to be given special treatment to avoid the
problems of excessive corrosion.
Figure 5.
Graph showing largest
mining leg cylinder
bore diameter against
approximate year of
introduction
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Nick A. Peppiatt
It should be pointed out that the longwall face advances intermittently, the frequency
being governed by the time the shearer takes to make a cut along the whole face. This
gives plenty of time for the seal and fluid to cool down between working strokes. The
shearer also uses hydraulic cylinders, but as these are used continuously, the operating
fluid here is generally mineral oil based. The volume of fluid in the shearer is very much
smaller than that required for the roof support system.
5. Typical seals and bearings for HFA fluids
5.1. Piston seals
The typical piston seal construction used in longwall roof support cylinders is shown in
figure 6. This has a polyester elastomer wear face, which is energised by a nitrile (NBR)
element, which seals between the piston and the face, and there are generally two hard
plastic anti-extrusion rings. This design has now become widely used in mining roof
support cylinders for two main reasons:
Figure 6.
Polyester faced piston
seal – 3-d model (left),
cross section shown
installed on a piston
(right)
1. It can run on a wide variety of surface finish values from skived and roller burnished
to honed tube bores of varying initial roughness and
2. It also works satisfactorily both in the older emulsion type fluids (HFAE) and the
newer micro-emulsions (HFAE) and synthetic fluids (HFAS), which were introduced
in the late 1980s and early 1990s to reduce the problems of soapy scums blocking
valves and filters [7].
Additionally this seal design can be readily fitted to one piece housings as the energising
rubber seal component and the polyester face can be stretched over the piston and the hard
plastic backup rings are split. The design has also proved adaptable to the growth in the
diameter of hydraulic leg cylinders from 250 mm to 500 mm over the last thirty years as
shown in figure 5.
As a result of the high inner stage pressures the seal is often fitted with additional support
rings to give the seal construction greater extrusion resistance.
Nick A. Peppiatt
131
5.2. Rod seals
Polyurethanes are well proven as extremely wear resistant materials in hydraulic applications using mineral oil. Unfortunately, some formulations are very susceptible to hydrolysis, or attack by water. The development of hydrolysis resistant grades with good resistance to permanent set has enabled polyurethane rod seal designs to be used successfully
in HFA water based fluids since the early 1990s. A typical design is shown in figure 7. In
this example, the polyurethane shell is energised by a nitrile (NBR) O-ring and the seal
also has a hard plastic anti-extrusion ring.
Figure 7.
Rod seal with hydrolysis
polyurethane shell – 3d
(left), cross-section shown
installed in a gland
(right)
5.3. Bearings
As is shown in the leg cylinder drawing, figure 3, plastic bearing strips are preferred
for guiding the piston through the bore and the rod through the gland. Such plastic
bearings have been used on piston heads for many years, but pick-up problems occurring
between cast iron glands and chromed rods as a result of the poor lubricity of HFA led
to their introduction for the glands as well. The bearing materials are typically acetal
(POM) or fabric reinforced polyester. The use of glass filled nylon (GFN) bearings is not
recommended as nylon (PA) is dimensionally unstable in the presence of water.
6. Reciprocating seal testing
The initial development of the polyester faced piston seals, which took place in the 1980s,
was carried out by the convergence testing of mining leg cylinders in the Hallite mining
leg cylinder test rig shown in figure 4. A full description of this testing was reported by
Peppiatt [6].
In addition, Hallite have carried out a considerable amount of wear testing in a standard
reciprocating rod seal test arrangement, as shown in figure 8. The test pressure used was
345 bar, which was constant as the rod was reciprocated. The rod speed was 0.2 m/s and
the maximum temperature allowed was 50 °C. The temperature was controlled by the
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Nick A. Peppiatt
dwell time at the end of each stroke. The number of cycles was 20,000. The tests clearly
showed that for the polyurethane rod seal material and the polyester elastomer piston face
material the wear over 20,000 cycles in HFA fluids is considerably less than in tap water.
Of course, if a test using otherwise similar conditions was run using mineral oil, a duration
of at least ten times greater would be expected.
Test seal grooves
►
►
►
Test pressure
Figure 8.
Cross-section through
a reciprocating rod
seal test pod. The rod
is reciprocated by an
external drive cylinder
7. Effects of corrosion
It is essential that the condition of a water based fluid system is monitored and the fluid
correctly maintained. The consequences of not doing this can be very expensive. Figure
9 shows the effects of using untreated mine-water in a roof support advancing ram where
the highly stressed surface of a skived and roller burnished cylinder tube has been attacked
and converted it from a benign surface for a seal to run on to one that is highly abrasive
and rough enough to rapidly destroy the piston seal and bearing.
Corrosion of the cylinder rods is much less of a problem because the rods are generally
hard chrome plated, often over a layer of bronze plate, which provides an impermeable
layer over the steel. In certain mine conditions, bronze plating only is used on the rods
of the leg cylinders. The leg cylinders of the roof support shield shown in figure 1 are an
example of this.
Nick A. Peppiatt
133
Figure 9.
Example of corroded
cylinder bore (above) and
resulting damaged piston
seal and bearing (below)
134
Nick A. Peppiatt
8. Seal material compatibility
A factor which must not be taken for granted is the chemical compatibility of fluids with
seal materials. ISO 6072 (9) gives details of suitable compatibility test conditions and
table 1 gives the results of such immersion tests for the polyester elastomer face material
and the polyurethane rod seal material in several water-based fluids, in comparison with
the ISO 6072 guidelines for allowable change. Note that these guidelines, which were in
the 2002 edition of ISO 6072, have now been removed from the 2011 edition. However,
they are retained in BS ISO 6072 as a National Annex.
From table 1 it can be seen that, although there are changes in properties of the materials
in distilled water and the HFAE fluids, these changes are not major, and the fluid/seal
material compatibility is acceptable. This agrees with experience, as these materials have
been used in these fluid types for many years without compatibility problems. On the
other hand major and unacceptable changes of material property have occurred with the
thickened HFA and the HFC water glycol. This again agrees with application experience.
Fluid
Distilled
water
Immersion
time and
temperature
Material
Change in
hardness
Volume
change (%)
Loss in tensile
strength (%)
Increase in
elongation (%)
HFAE
HFAE Thickened
microHFA
emulsion
(20/80)
56 days at 70 °C
HFC
(water
glycol)
ISO
6072
guideline
42 days at 70 °C
A
B
A
B
A
B
A
B
A
B
-2
0
-1
0
-2
0
-4
-1
-2
0
+10/-10
1
1
7
2
5
4
18
12
5
3
-5/+20
13
14
3
12
27
4
54
84
77
100
50
12
15
41
59
20
21
41
-98
-28
-100
-50
A = Hydrolysis resistant polyurethane (used in rod seals)
B = Hydrolysis stabilised polyester elastomer (used in piston seal faces)
Table 1.
Compatibility
results for hydrolysis
resistant polyurethane
and polyester seal
materials in various
water based fluids
9. Use of HFCs
HFC fluids are polymer solutions (generally glycol based) of between 35 and 80 % water.
They can have viscosities similar to mineral based hydraulic oils and the anti-freeze
properties of the additive allow them to work down to -20 °C. The upper temperature is
still limited by the high vapour pressure of water. They are commonly used where a fire
resistant fluid is working harder and needs to provide a better lubricity than HFA. They
are commonly used in applications such as hydraulic systems for steel works. There is
Nick A. Peppiatt
135
a common misconception that, because they are fire-resistant fluids, they are also fluids
which can operate at higher temperatures. This is clearly not desirable because of the low
vapour pressure of water and seal material compatibility concerns.
It can be seen from table 1 that the seals developed for longwall mining applications may
not be successful when used in HFC fluids, because of the problems of long-term chemical
attack. In general, nitrile rubber seals are to be preferred [8].
10. Conclusions
Going back to the nineteenth century, the original hydraulic fluid was plain water. Now,
only a very small proportion of hydraulic systems use plain water as a fluid. This is
because water on its own has a number of major disadvantages. A number of these can be
overcome by the addition of a low percentage of additives (HFA), and these do not reduce
the fire resistance of the media compared with plain water. Such fluids have been used
successfully for many years in underground hydraulic equipment to extract minerals – an
application where fire resistance is paramount. Seals have been successfully developed
for this application to enable the output of such equipment to increase many times. The
importance of maintaining the condition of such fluids to achieve the successful operation
of this mining equipment cannot be overemphasised.
There are several different water based fluid types and seals that are suitable well proven
for one type may not be suitable for another.
References
[1] SO 7745 Hydraulic fluid power – Fire-resistant (FR) fluids – Requirements and
guidelines for use, 2010.
[2] I. McNeil ‘Hydraulic power transmission- the first 350 years’. I.Mech.E. The
Newcomen Society lecture 26th November 1975.
[3] C.R. Burrows ‘Fluid power systems design – Bramah’s legacy’. Proceedings of
I.Mech.E. 13th December 1995.
[4] ‘Water Hydraulics – a Technical Guide’ BFPA P82, 1999.
[5] ISO 6743-4, Lubricants, industrial oils and related products (class L) – Classification
– Part 4: Family H (Hydraulic systems), 1999.
[6] N.A.Peppiatt ‘New developments in hydraulic sealing for longwall roof supports’.
Paper presented at 11th International Conference on Fluid Sealing, Cannes, 8th -10th April
1987.
[7] N.A.Peppiatt ‘Reciprocating Hydraulic Seals for Water-Based Fluids’. Paper presented
at NFPA 47th Conference on Fluid Power, 23rd-25th April 1996.
[8] N.A. Peppiatt. ‘Sealing water-based fluids’ Paper presented at 21st International
Conference on Fluid Sealing, Milton Keynes, 30th Nov-1st Dec 2011.
[9] ISO 6072 Rubber – Compatibility between hydraulic fluids and standard elastomeric
materials, 2011.
136
Nick A. Peppiatt

Hydraulic seals for water and water-based fluids

Transcription

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