Valve

A valve for regulating the flow of a liquid, for example in liquid chromatography, comprises a valve seat and a valve ball which are made of electrically conductive ceramic or glass material. Due to the electric conductivity, electrostatic charging is avoided which would otherwise attract dirt particles and lead to contamination and leakage of the valve. The valve is chemically inert and is not corroded by the solvents typically used in analytical chemistry.

The invention relates to a valve for regulating the flow of a liquid, with 
at least two sealing elements, for example a valve seat and a valve ball. 
Such valves are used, for example, in pumps for analytical measuring 
instruments such as liquid chromatographs. 
BACKGROUND OF THE INVENTION 
Liquid pumps in use with analytical measuring instruments require a high 
flow accuracy. In order to meet this requirement the pumps must be 
equipped with precision valves. Usually ball and seat type valves are 
used. Passive valves, wherein the valve ball is lifted by the flow of 
liquid, are known, for example, from U.S. Pat. No. 3,810,716 or from U.S. 
Pat. No. 4,139,469. The valve balls are often made of ruby and the seat of 
sapphire. Valves, wherein the ball and seat are composed of aluminum oxide 
ceramics (Al.sub.2 O.sub.3), are also known. Such ceramic valves are 
described in U.S. Pat. No. 4,832,075 or in U.S. Pat. No. 4,862,907 or in 
U.S. Pat. No. 5,002,662. These ceramic valves produced by the die-pressing 
method have economic and other advantages over ruby/sapphire valves. 
Both the ruby/sapphire and the Al.sub.2 O.sub.3 -ceramic/Al.sub.2 O.sub.3 
-ceramic combinations have considerable advantages in use with a pump for 
high-pressure liquid chromatography. These advantages are: resistance to 
wear, chemical resistance to commonly used solvents, accuracy of form for 
good sealing properties, a high machining quality at economically 
efficient manufacturing costs. In high-pressure liquid chromatography it 
is possible, especially where ruby/sapphire valves are being used, that 
under the influence of acetonitrile and water a coating of organic 
substances forms on the valve material. As a result of this coating the 
ball will adhere to the seat and the surface of the seat will become 
contaminated. This problem can be reduced by using Al.sub.2 O.sub.3 
ceramics instead of ruby or sapphire. 
Besides the mentioned passive valves, active valves may also be used in 
pumps for analytical measuring instruments, wherein the valve ball can be 
activated by a valve actuator. Such an active valve is known from EP-A-0 
328 696. Active valves have the advantage that no flow reversal is 
required to close the valve. Further, movement of the valve elements is 
largely independent of external influences. Either ceramics or 
ruby/sapphire can be used as materials for the ball and seat. 
The disadvantage of the valves as described above is their great tendency 
to become dirty. In particular in high-pressure liquid chromatography, it 
is impossible to avoid that smallest particles of dirt are deposited. 
These particles either originate from supply vessels, become detached from 
the feed system or are solids entrained in the solvent. These particles of 
dirt can build up on the valve. Once sufficient dirt has accumulated on 
the sealing surface the valve ceases to be tight and must be replaced. 
SUMMARY OF THE INVENTION 
It is thus an object of the invention to create a valve of the type 
mentioned above, wherein the sealing elements, for example the ball and 
seat in a ball valve, are prevented from becoming contaminated. 
It is a further object of the invention to provide a valve which is 
chemically inert so that it is not corroded by the solvents typically used 
in analytical chemistry, and which has a long service life. 
It is an additional object of the invention to provide a valve which can be 
manufactured with high precision in an easy way at a comparatively low 
price. 
These and other objects of the invention are met by a valve as defined in 
the independent claims. According to an underlying principle of the 
invention, the sealing elements, such as a valve seat and a valve ball, 
comprise ceramic or glass material which is electrically conductive. The 
electric conductivity is achieved either by addition of substances to the 
usual ceramic or glass base material or by providing an electrically 
conductive coating on the ceramic or glass material. 
The invention is based on the finding that the mentioned contamination of 
the prior art valves arises from the fact that the sealing elements, for 
example the valve ball and the valve seat, are electrostatically charged 
by fluid friction and thus cause electrostatic forces to act on the 
particles of dirt present in the liquid, whereby these particles are 
attracted and accumulate on the valve. The invention prevents 
electrostatic charging by selecting an electrically conductive ceramic or 
glass material for the sealing elements. The sealing elements therefore do 
not cause electrostatic forces to act on particles of dirt, with the 
result that these do not accumulate on the valve and the valve remains 
tight. 
The contamination problem is therefore solved and at the same time the 
positive characteristics of a ceramic or glass valve as described above 
are retained. In particular, a valve, according to the invention has a 
longer service life than conventional valves. 
A further advantage of the invention is that the sealing elements, 
especially the valve seat, can be manufactured more economically than is 
the case with valves made of sapphire. According to the an embodiment of 
invention using ceramics, powder is used as a starting material, which 
enables the basic shape of a seat to be directly sintered. That is to say, 
the powder (with a particle size in the micrometer range)is placed in a 
metal die which is roughly the negative of the finished part. This means 
that the cost of the subsequent grinding process is reduced considerably 
compared with the manufacture of a sapphire seat. In an economically 
advantageous way, 100 or more parts can be pressed and sintered at the 
same time in a metal die. 
In an embodiment of the invention the electrically conductive ceramic 
material is composed of a mixture of aluminum oxide (Al.sub.2 O.sub.3) and 
titanium carbide (TIC). The conductive ceramic material is preferably 
manufactured by sintering and subsequent further densification at high 
temperatures and under pressure. A valve according to the invention can, 
for example, be designed as an active valve with an actuator for 
activating a sealing element, for example the valve ball, or as a passive 
valve. In either case, the invention prevents particles of dirt from 
accumulating on the valve, which would result in loss of tightness.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the valve according to an embodiment of the invention 
comprises an inlet port 18 and an outlet port 19. The inlet port can, for 
example, be connected to a liquid supply vessel, by screwing a connecting 
tube on to the thread shown. The outlet port 19 can, for example, be 
connected to a pump. A preferred application for the valve according to 
the invention is in the field of high-pressure liquid chromatography where 
the inlet port 18 is connected to a vessel containing solvents or liquid 
samples and the outlet port 19 is connected to the inlet of a 
high-pressure pump. 
Inside the valve liquid can flow from the inlet port 18 along the 
connecting channel 20 into the cavity 21 which is sealed on one side by a 
diaphragm 10. Liquid can flow from this cavity through a further 
connecting channel 22 and a ball valve (if open), which comprises a ball 2 
and a seat 1, to the outlet port 19. 
The valve ball 2 is held in place in the seat 1 by a preloaded spring 3. 
The valve seat 1 is clamped in position between an insert 4 and a valve 
body 5. 
Sealing rings 6 and 7 are provided between the insert 4 and the seat 1 and 
between the valve body 5 and the seat 1. The insert 4, together with the 
seat 1 and the ball 2, the spring 3 and the sealing ring 6 are pressed 
into the valve body 5 so that the seat 1 is in contact with the sealing 
ring 7. Instead of the press fit as described above, the insert 4 and the 
valve body 5 could also be welded, cemented or screwed together. 
In FIG. 1 the ball valve is shown in the closed position. The valve-can be 
opened by means of an activating stem 8 which extends from the cavity 21 
through the connecting channel 22 and the seat 1 to the ball 2. The 
activating stem is shown in detail in FIG. 2. As can be seen, the stem 8 
consists of a head end 40, a middle section 41 and a tail end 42. The head 
end 40 is provided with a recess 43. This recess ensures that liquid can 
flow through the connecting channel 20 and the cavity 21 into the 
connecting channel 22, even if the stem 8 is pressed against the valve 
body 5. The part of the stem which extends into the connecting channel 22 
is rectangular in cross-section and the longer side is somewhat smaller 
than the cross-section of the connecting channel 22 so that the stem can 
be moved inside the connecting channel. The activating stem 8 tapers 
towards its lower end and passes through the seat 1 until it touches the 
valve ball 2. The diameter of the tail end 42 has been chosen to permit 
liquid to flow between the stem and the internal wall of the seat. 
The purpose of the screwed end 17 and the retaining ring 23 is to enable 
the valve to be flange-mounted on a pump head. The seal 24 produces a 
positive connection between the screwed end 17 and the valve body 5. As 
previously mentioned, the cavity 21 in which the head of the activating 
stem 8 is located is sealed by the diaphragm 10. The diaphragm is clamped 
in position in a V groove between an adapter plate 9 and the valve body 5. 
On the other side of the diaphragm 10 is a rubber disc 11 which serves as 
a cushion plate when the stem 8 is being actuated. 
The stem 8 is actuated by a switching magnet 12, which is connected to the 
adapter plate 9. The switching magnet consists of a housing 13, an 
armature 14, a spring 15 and a limit stop 16. The coil for activating the 
switching magnet is located inside the housing 13. When the coil is 
activated, the armature 14 is pressed against the spring 15. Thus, no 
force is exerted on the stem 8, with the result that the spring 3 presses 
the valve ball 2 into the seat and the valve is closed, In order to open 
the valve the switching magnet is deactivated so that the armature 14 
presses against the rubber disc 11. In this way the stem 8 is displaced 
and the ball 2 is lifted clear of the seat. 
According to an important aspect of this embodiment of the invention the 
valve ball 2 and the valve seat 1 are composed of an electrically 
conductive ceramic material. An example of such conductive ceramic 
material is a mixture of aluminum oxide (Al.sub.2 O.sub.3) and titanium 
carbide (TIC). In one embodiment the proportions are approximately 10% TiC 
and 90% Al.sub.2 O.sub.3. This conductive ceramic material has an 
electrical resistivity of approx. 2.1 .multidot.10.sup.-3 Ohm.cm. This 
makes it possible to prevent the aforementioned contamination problem. In 
this particular embodiment (10% TiC and 90% Al.sub.2 O.sub.3) the ceramic 
material has a density of 4.2 g/cm.sup.3 and its crystallite size is less 
than 3 micrometers. 
In order to produce the conductive ceramic material the components, in this 
example Al.sub.2 O.sub.3 and TiC are sintered densely and further 
densified at high temperature while under pressure. The material thus 
obtained is then finished, in particular by grinding, to produce the ball 
and seat. This finishing process is comparable with the production of the 
ball and seat from sapphire and ruby respectively. As an alternative to 
the production process as described the ball and seat can also be sintered 
in approximately the desired form although the above mentioned further 
densification at high temperature while under pressure must also be used 
in this case. 
Of course, other conductive ceramic materials besides Al.sub.2 O.sub.3 and 
TiC can also be used. Other examples are SiC with free Si or SiC with TiB. 
The following general rule applies to electrical resistance: the smaller 
the electrical resistance the less the component will be electrically 
charged and accumulate dirt as a result of static electricity. 
In a further embodiment of the invention, the valve has substantially the 
same design as shown in FIGS. 1 and 2, but the valve ball and the valve 
seat are made of electrically conductive glass instead of electrically 
conductive ceramics. Such valves of electrically conductive glass have 
substantially the same advantages as those made of conductive ceramics 
(see above). An example for electrically conductive glass is the glass 
sold by the company Schott under the name "S 8900". The glass is doped 
with iron ions (Fe 2+ and Fe 3+ ions). As an alternative to the ball valve 
design, a design with a plate and a hole manufactured from conductive 
glass can be used. The use of glass material allows to apply alternative 
manufacturing processes, such as etching processes. Furthermore, smaller 
dimensions of the sealing components can be achieved with the glass 
technology. 
According to a still further embodiment of the invention, the material for 
the valve ball and the valve seat is a substantially non-conductive 
ceramic or glass material which is covered at the surface by a layer of 
electrically conductive material. An example for such an electrically 
conductive material is gold. The gold layer is applied, for example, by a 
sputtering method. Further examples of materials used as a conductive 
surface layer are SnO.sub.2 and InSnO.sub.2 ; these are preferably applied 
on glass material. With the latter substances conductivities up to 20 
Ohm/cm can be achieved. 
The valve shown in FIG. 1 is an active valve wherein the valve ball is 
pushed out of the seat by a valve actuator. In an alternative active valve 
the valve seat could move and the valve ball remain static. 
It is understood that the invention is not only for use in active valves 
but also in passive valves, in particular check valves wherein the ball is 
lifted by the flow of liquid. The use of ceramic or glass materials which 
are either conductive or covered with a conductive layer in such valves 
prevents particles of dirt from accumulating on the valve and impeding its 
operation. These check valves may be of the single seat or multiple seat 
type, i.e. provided with several ball/seat assemblies. Such a multiplicity 
of serially connected ball/seat assemblies can be arranged in a single 
housing or in separate housings, connected via a tube. Furthermore, 
ball/seat assemblies according to the invention could be connected in 
parallel. Such a parallel arrangement can be useful if the pressure drop 
across a single valve becomes too high. 
It is also understood that the above mentioned arrangements of ball/seat 
assemblies of the invention, i.e., serial or parallel connection of 
several valves, is not limited to check valves, but can also be used with 
active valves. 
The invention can be used to particular advantage in analytical 
instruments, especially those used in liquid chromatography. In this 
equipment accurate flow control and therefore the precise operation of the 
valves is very important for accuracy of measurement. However, it is also 
understood that the invention is not restricted to this field of 
application.