Harwood
Engineering Company, Inc.
A Rotating Valve for High Pressure
by P.H. Clement, L.H. Abbot, and D.H. Newhall
Some operations of tumbling barrel type mixers require that the vessel
be isolated during treatment of the charge. The present paper describes
a valve designed and constructed for such a purpose. The valve is attached
co-axially to and rotates with the vessel, is capable of containing gas
or liquid pressures of 20,000 psi, and is operated by instrument air applied
through a slip joint.
Introduction
The tumbling barrel is one of the simplest but most useful types of mixer
or reactor. Essentially it consists of a container, usually cylindrical,
rotating about a shaft, usually horizontal, which may or may not be co-axial
with the cylinder. It is particularly adapted where a solid and either
a liquid or gaseous phases are involved. In some cases a fixed quantity
of solid within the reactor is treated by a fixed amount of fluid. In others
the solid is exposed to a continuous flow of the fluid. In both cases appropriate
valving is required, either for quantity of flow control of the fluid.
The present paper describes a valve designed for such a system.
Function of Valve
The prime requirement of this valve, shown in Figure
1, is that it is capable of isolating a rotating reactor vessel pressurized
to 20,000 psi for normal operation, 30,000 psi for test. Since it is not
feasible to use a slip coupling between the compressor and the reactor
at these pressures and with continuous rotation of the vessel it is necessary
to integrate the reactor and the valve: that is, the valve is fastened
rigidly to the reactor. After the reactor is pressurized the valve is closed,
pressure in the coupling and compressor connection is released, and rotation
may proceed.
Description of Valve
The valve is shown completely by the accompanying Figure
2. The valve body, V, is connected rigidly through a nipple,
Nv, on one side to the reactor and on the other through a slip joint
and nipple Np, to the pump or compressor. The valve stem, P,
makes up against the seat, S, by the action of the actuator A.
The weight of the valve is supported by pillow blocks, B; and guide
bearings, G, provide for the rotation of the sleeve, L, about the
nipple, Np. A weight, W, serves to counter balance against
the actuator. A slip joint with rotating inner ring, J1, and stationary
outer ring J2, permits 40 psi instrument air to be applied to the
actuator through tubing, T. A rupture disk, R, is conveniently
located in the valve body below the valve seat. Similarly, it was advantageous
to attach the pressure sensor, M, for the reactor, to the side plate,
N. Connection to the pressure sensor is made through the pressure
tubing, C.
Materials
In the selection of materials the designer must envision the use with reagents
which might be corrosive or otherwise deleterious, as well as capability
of withstanding the stress to be encountered. For the valve body it was
decided to use a block of A-286, which combines high physical properties
with superior resistance to severe environments. Some difficulty is encountered
in machining a block of this material, but adequate techniques are now
available to cope with this problem and a block machined to the required
tolerances was produced without undue hindrance.
It is customary in this type of valve to select a valve stem or needle
of somewhat greater hardness than the seat, which can be re-figured if
necessary after long use. Accordingly, in this case the valve stem is made
of 440 stainless steel heat treated to RC 58-62 whereas the beryllium-copper
seat is RC 39-44.
Actuator
The valve actuator features a diaphragm exposed to instrument air whose
direction drives a plunger with a snap action to open or close a valve.
As here applied the plunger displaces a toggle joint, F, in Figure
2, which in turn displaces the valve stem, P. This technique
has been utilized very effectively in high pressure practice.
Operation
To illustrate the degree of pressure control possible with a device of
this nature it is to be noted that Harwood Engineering Company has been
able to control 200,000 psi using instrument air at 40 psi in this manner.
In other applications air at 20 psi has isolated 125,000 psi system pressure.
The actuator unit is provided with stops which can be set for suitable
limits of displacements.
As noted previously, the air supply is transmitted through the slip
joint, J1, J2. This is dependent on the nature of the seal between
the slip rings, J1 and J2. It turns out that Bal-Seal graphite-filled
Teflon withstands absolutely this requirement — stationary on one side,
rotating on the other, and accommodating 50 psi air pressure.
The valve is normally open. For closure, action is necessary against
a restraining force. This is provided by the Belleville spring, Q,
in addition to the reactor pressure and a certain amount of friction. Proportions
are such that the displacement of the valve stem in closing against the
seat deforms the Belleville washers well within their elastic limit.
Special Features
Certain features deserve special attention. The packing at K must
hold full pressure, 20,000 psi, when the reactor is pressurized, but after
the valve is closed pressure on the pump side of the valve is released
and the packing is no longer under more than nominal stress. The packing
is, however, subject to the friction caused by the rotation of the valve
body about the stationary nipple. The packing consists of a graphite-filled
Teflon primary seal ring between copper-beryllium wedge and keeper rings.
The service is severe, since the packing must withstand continuous rotation
for long periods without erosion. The seal has given very gratifying service.
On the other hand, a seal between the rotating inner ring and the stationary
outer ring, J1 and J2, is subjected to instrument air pressure,
40 psi, throughout the period of rotation. A graphite-filled Teflon seal
ring has given complete satisfaction in this requirement.
The pressure sensor, M, consists of four strain gages connected
to form a bridge and cemented to a stress member subjected to reactor pressure.
Four leads are brought out to a pressure indicator-controller, passing
under the bearing, B, and through a slip ring, not shown, mounted
on the rotating axis between the valve and the reactor vessel. The signal
from the bridge must not be impaired in the slightest by any parasitic
resistance rings. For this service it was possible to find a commercial
slip ring which has given perfect performance.
The most difficult requirement specified that the reactor unit rotate
without binding. This imposed very close tolerances on all dimensions affecting
co-axiality. In addition a long process of shimming up and adjusting was
necessary. It turned out, however, that the specification was too lenient.
Though binding could not be sensed, leakage occurred during rotation, in
the connections between the valve and the vessel. This was finally eliminated
upon the attainment of perfect alignment and co-axiality as shown by sensitive
dial indicators.
Conclusion
This subject valve has now been in operation for somewhat over a year.
Since the overcoming of leakage in the setting-up period, performance appears
to have been completely satisfactory. In particular, the utility of graphite-filled
Teflon under severe conditions of relative motion has been further demonstrated.
In conclusion, this paper announces the availability of a valve capable
of effectively isolating a rotating action vessel during its operation
at high pressure.
Harwood Engineering Company, Incorporated – © June, 1999
455 South Street, Walpole, Massachusetts 02081
phone: 508-668-3600
fax: 508-660-2276
http://www.ultranet.com/~harwood/
email: harwood@ma.ultranet.com