Non-locking tapered plug valve

A non-locking tapered plug valve is provided which prevents taper locking of the plug member by ensuring against pressure responsive plug movement toward the small extremity thereof due to positive or negative hydraulic pressure transients. The chambers at each axial extremity of the plug member are of such volumetric relation during positive pressure transients that the chamber at the small extremity of the plug reaches equilibrium with flow passage pressure before the chamber at the large extremity of the plug member thus ensuring that any pressure responsive resultant force acting on the plug member is directed toward its large extremity. To avoid taper locking during a negative pressure transient, an equalization passage with a check valve is provided to permit unidirectional flow from the large extremity of the valve chamber to the main flow passage. This allows a quicker equalization of pressure between the large extremity of the valve chambers with line pressure, whereas the higher pressure in the small extremity of the valve chamber creates a force on the plug member away from the taper locking direction.

FIELD OF THE INVENTION 
This invention relates generally to tapered plug valves which are utilized 
for controlling the flow of fluids through flow lines. More specifically, 
the present invention is directed to a plug valve mechanism which 
effectively prevents pressure locking of the plug member in the body 
cavity of the valve by the effects of positive and negative pressure 
transients and prevents locking of the plug member by the effects of 
temperature transients. 
BACKGROUND OF THE DISCLOSURE 
Various types of plug valves, such as cylindrical plug valves, ball valves 
and tapered plug valves have been utilized for a significant period of 
time. Tapered plug valves are favored in many service conditions because 
of the closeness of fit that can be obtained between the tapered plug 
member and the tapered valve chamber surfaces. Tapered plug valves offer 
the capability for achieving close sealing tolerances without the 
introduction of significant torque to achieve valve operation. Tapered 
plug valves are available in lubricated models where sealant material 
provides for lubrication of internal components and assists in the sealing 
activity of the valve. The injected lubricant minimizes leakage in the 
clearance between the tapered plug member and the internal tapered sealing 
surface of the valve body. Tapered plug valves are also available in 
non-lubricated models where sealing is affected by the closeness of fit 
between the plug and body surface. 
Tapered plug valves are at times disadvantageous because pressure induced 
locking may occur especially under the influence of positive and negative 
pressure transient conditions. In cases where surges in line pressure 
occur or where line pressure suddenly increases or decreases, the tapered 
plug member can be moved axially under pressure influence to a tightly 
wedged position within the apex tapered portion of the valve chamber. In 
this tightly wedged position, rotation of the plug member may not be 
possible or excessive torque may be required to move the plug member 
between its open and closed positions. The relationship of the plug member 
to internal valve chamber walls typically defines chambers at each axial 
end of the plug members. When line pressure changes occur, a pressure 
differential exists between these internal chambers which becomes balanced 
at the end of a "transient period." The transient period is that period of 
time during which there is a transfer of fluid through the clearances 
between the plug and body to the internal chambers due to the pressure 
differential. Pressure transients are "positive" when line pressure 
increases causing leakage flow from the flow passage to the internal 
chambers and "negative" when line pressure decreases, causing leakage flow 
from the internal chambers to the flow passage. It is desirable therefore 
to provide means for preventing pressure responsive movement of the plug 
member to a locked position within the valve chamber during either 
positive or negative pressure transients. 
As temperature transients occur the valve body may increase in effective 
dimension thereby enabling the tapered plug member to move further towards 
the small end of the valve chamber. Upon subsequent decrease in valve 
chamber dimension, caused by cooling by the valve body, the valve chamber 
surfaces of the valve body may seize the plug the lock it against 
rotation. It is desirable of course to eliminate the possibility of plug 
movement toward its apex and thus prevent locking of the plug member due 
to the effects of temperature transients. 
Taper locking can also occur under circumstances where the valve is mounted 
with the large base portion of the plug member positioned above the small 
apex portion of the plug. The weight of the plug can cause it to descent 
toward its taper apex by gravitational force and in time it can become 
locked against normal rotation. 
PRIOR ART 
Various valve improvements have been introduced to minimize the effects of 
hydraulic locking in tapered plug valves. One such improvement is 
identified in U.S. Pat. No. 4,034,776 of Eshghy which discloses a tapered 
plug member defining a passage communicating the large end of the valve 
chamber with the flow passage of the valve. An opposing passage is also 
provided in the plug member which is closed by a free-floating ball check 
member 110 which prevents lubricant material flow from chamber 76 to the 
flow passage of the valve. A spring member also urges the tapered plug 
member towards its large extremity. U.S. Pat. No. 4,135,544 of MacLeod 
discloses a balanced tapered plug valve wherein the pressure responsive 
area at the small end of the plug member is greater than the pressure 
responsive area at the large end. A resultant force is therefore developed 
during the negative transient which urges the plug member towards its 
large end. U.S. Pat. No. 4,174,092 discloses a tapered plug valve having a 
passage 98 in the plug member which balances pressure of chamber 78 with 
the flow passage of the valve. Also, the valve stem is urged by body 
pressure to a position compressing the stem seal 27. The plug of this 
valve is not positively biased in a definite pressure responsive 
direction. Rather, the valve is of pressure balanced design and the plug 
member is not generally movable to a locked position responsive to 
pressure transients. The plug, however, is sensitive to minute pressure 
differential and therefore is susceptible to axial shifting back and forth 
during period of pressure transient induced forces. The prior art patents 
noted above do not disclose tapered plug valve apparatus having the 
capability of preventing plug locking responsive to both positive and 
negative hydraulic pressure transients. 
SUMMARY OF THE INVENTION 
It is therefore a primary feature of this invention to provide a novel 
tapered plug valve mechanism having a favorable pressure responsive bias 
toward the large extremity thereof during both positive and negative 
pressure transients thus obviating any possibility of pressure locking of 
the plug member. 
It is also an important feature of the present invention to provide a novel 
tapered plug valve mechanism which prevents pressure responsive movement 
of the tapered plug member toward the taper apex of the valve chamber 
responsive to both positive and negative pressure transients. 
It is also a feature of this invention to provide a novel tapered plug 
valve mechanism of the lubricated or nonlubricated type which employs 
pressure responsive means generating a favorable force bias during 
positive and negative pressure transients which urges the tapered plug 
member toward its large or base extremity. 
It is also a feature of this invention to provide a novel tapered plug 
valve mechanism which prevents tapered plug movement toward the taper apex 
of the valve chamber during conditions of temperature transients, thereby 
effectively preventing plug locking as the valve chamber dimension 
decreases responsive to temperature change. 
It is an even further feature of this invention to provide a novel tapered 
plug valve mechanism which is not subject to plug locking when the valve 
is installed with the large end of the plug member located above the 
small, taper apex end of the plug member which might otherwise permit 
gravitational movement of the plug member into the apex taper of the valve 
chamber to a position where the possibility of taper locking can occur. 
The foregoing and other features and advantages of this invention are 
attained by a non-locking tapered plug valve mechanism incorporating a 
tapered plug member rotatably mounted within a corresponding tapered valve 
chamber defined within a valve body. The plug member defines a passage 
communicating the flow passage of the valve with a chamber at the large or 
base extremity of the plug member. This pressure balancing passage is 
provided with a check valve permitting unidirectional flow from the base 
chamber to the flow passage and preventing flow from the flow passage to 
the base chamber. The check valve is urged by a compression spring into 
seated relation with a valve seat defined within the passage. The valve 
mechanism also incorporates a compression spring system mechanically 
urging the plug member toward the large or base extremity thereof. 
When a negative pressure transient occurs due to rapid decrease in flow 
passage pressure, pressure within the base chamber at the large end of the 
plug member is vented past the check valve into the flow passage thereby 
preventing pressure induced movement of the plug member toward its apex 
extremity. Pressure within the apex extremity of the valve chamber at the 
small end of the plug member maintains a favorable force bias urging the 
plug member toward its large extremity, away from a possible taper locking 
position. 
Positive pressure transients due to sudden increase in line pressure acting 
upon differential plug areas, larger at the large extremity of the plug 
member, develop a resultant force urging the plug member toward its large 
extremity again away from a possible taper locking condition. 
Additionally, the relative volumetric dimensions of the base chamber and 
apex chamber are so correlated with the clearance areas between the plug 
and body such that the apex chamber becomes equalized with line pressure 
before equalization of the base chamber during a positive line pressure 
transient. This ensures that the only pressure induced resultant force 
acting upon the plug member will be a favorable force bias in a direction 
toward the base extremity of the tapered plug member.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings and first to FIG. 1 a non-locking tapered 
plug valve is illustrated generally at 10 which incorporates a valve body 
structure 12 defining a frustoconical or tapered internal valve chamber 
14. The valve body also defines a flow passage 16 intersecting the valve 
chamber and providing for flow of fluid through the valve. The valve body 
further defines connection flanges 18 and 20 for bolted connection of the 
valve to corresponding flanges of a flow line. Although flanges are shown 
for bolting connection, such as not intended to limit this invention, it 
being obvious that other means for connection of the valve body to a flow 
line may be utilized without departing from the spirit or the scope 
hereof. 
The valve body defines an access opening 22 which is closed by means of a 
bonnet structure 24. The bonnet 24 may be secured to the body 12 by bolts 
or studs 26 or it may be secured to the body structure in any other 
suitable manner. 
Within the valve chamber is located a frustoconical or tapered plug member 
28 which defines a tapered external sealing surface 30 establishing mating 
sealing engagement with a correspondingly tapered internal sealing surface 
32 defining a portion of valve chamber 14. For purposes of simiplicity the 
valve body and plug assembly are shown to be of the lubricated type; 
however it is intended that plug valves of nonlubricated character will be 
within the scope of this invention as well. A lubricant injector 33 is 
connected to the valve body at a lubricant injection passage 35 for 
injection of lubricant into the valve chamber. Internal lubricant channels 
(not shown) are defined for effective distribution of lubricant between 
the plug and body surfaces, as well as the upper and lower chambers 
C.sub.2 and C.sub.3. 
At the lower or larger end of the valve mechanism is provided an adjustment 
screw 34 which is received by a threaded opening 36 in the bonnet member 
24. The adjustment screw, which is sealed with respect to the bonnet 24 by 
a circular seal member 37, provides an upper support surface 38 which is 
in supportive engagement with the lower extremity 40 of the plug member 
28. The adjustment screw 34 is utilized to achieve optimum positioning of 
the tapered sealing surface 30 of the plug member relative to the tapered 
internal sealing surface 32 of the valve body to thereby insure optimum 
sealing capability. As wear occurs during operation of the valve the 
adjustment screw may be manipulated while the valve is in service to shift 
the plug member 28 toward its apex taper (small extremity) to reestablish 
optimum sealing capability. The adjustment screw member 34 also functions 
as a stop to limit movement of the plug member toward its base (large 
extremity). 
To impart rotation to the plug member 28 within the valve chamber 14 a 
valve stem 42 is provided which extends in sealed relation through a valve 
stem opening 44. The valve stem 42 is provided with an intermediate 
enlargement 46 which engages inner surface 48 of the valve chamber and 
functions as a stop to prevent the stem from being blown from the valve 
opening by pressure within the valve chamber. The valve stem is also 
provided with a drive portion 49 establishing driving relation with the 
plug member 28 such that, upon rotation of the valve stem 42, 
corresponding rotation of the plug member is achieved. The drive portion 
49 is preferably of non-circular structure and is received within a 
corresponding non-circular drive receptable 50 defined in the apex 
extremity 52 of the plug member. 
As mentioned above, at times temperature transients can cause temporary 
enlargement of the valve chamber due to metal expansion. When this occurs, 
especially if the valve is mounted with its base extremity above the apex 
extremity, the plug member has a tendency to move under the influence of 
gravity toward the apex extremity of the valve chamber. Upon subsequent 
cooling of the valve body, shrinkage of the metal can effectively reduce 
the dimension of the valve chamber causing the tapered internal sealing 
surface 32 to seize the plug member and prevent or retard its rotation. To 
prevent the possibility of this occurrence a compression spring assembly, 
illustrated generally at 54, is interposed between the enlargement 46 of 
the valve stem and the apex extremity 52 of the plug member. The spring 
assembly continuously urges the plug member 28 toward its base extremity. 
Under conditions where effective valve chamber enlargement occurs due to 
temperature transients, the force of the spring assembly will prevent the 
plug member from moving to a possible locking position within the valve 
chamber. If desired, the spring assembly may be formed by a pair of 
bellville springs 56 and 58 or, in the alternative, any other suitable 
compression spring system may be employed to maintain the plug member in 
engaged relation with the support surface 38 of the adjustment screw 34. 
As discussed above, it is a primary feature of this invention to provide a 
tapered plug valve mechanism having an effective means to prevent plug 
taper locking due to both positive and negative pressure transients. 
According to the present invention such may be conveniently accomplished 
by providing controlled communication between the base portion of the 
valve chamber and the flow passage of the valve and correlating clearance 
area between the plug and valve chamber surfaces. The flow passage 16 
effectively defines a chamber C.sub.1. The plug member 28 cooperates with 
the valve chamber 14 to define an apex chamber C.sub.2 at the small end of 
the valve chamber, having a volume V.sub.2 and a base chamber C.sub.3 at 
the large end of the valve chamber, having a volume V.sub.3. A flow path 
exists at the interface clearance between the sealing surface of the plug 
and valve chamber. The interface clearance is further characterized as the 
clearance leakage path between the flow passage and the apex chamber 
C.sub.2 which is represented by flow path area a.sub.2 and the clearance 
leakage path between the flow passage and the base chamber C.sub.3 which 
is represented by flow path area a.sub.3. Flow path areas will differ 
principally due to the differing diameters resulting from the taper of the 
plug and body surfaces. Thence, if chambers C.sub.2 and C.sub.3 were of 
equal volume their pressure would become balanced with line pressure at 
differing rates because of the difference in clearance areas forming flow 
path areas a.sub.2 and a.sub.3. 
Upon a sudden change in line pressure (positive or negative) such as would 
occur upon opening and closing of the valve or upon opening or closing of 
another valve in the line or upon rupture of the line either upstream or 
downstream, the chambers C.sub.2 and C.sub.3 would be at different 
pressures as compared to line pressure in chamber C.sub.1. Fluid transfer 
in the clearance flow paths will immediately begin. After a period of time 
due to such fluid transfer, the pressure within chambers C.sub.2 and 
C.sub.3 will become balanced with the pressure in chamber C.sub.1 defined 
by the flow passage of the valve. 
Upon a sudden rise in pressure within the flow passage chamber C.sub.1 a 
pressure differential will exist between flow passage pressure and the 
pressure of chambers C.sub.2 and C.sub.3. At this point it should be noted 
that the plug member 28 and its relationship with the valve chamber of the 
valve body defines pressure responsive areas A.sub.2 and A.sub.3 with area 
A.sub.3 being larger than area A.sub.2. Pressure P.sub.1 within flow 
passage C.sub.1, acting upon plug areas A.sub.2 and A.sub.3 will develop a 
resultant force acting to urge the plug member toward its larger 
extremity. As fluid transfer occurs past the plug member through the 
annular clearance area a.sub.2 and a.sub.3, the chambers C.sub.2 and 
C.sub.3 will, after a time, reach balanced pressure with flow passage 
pressure and conditions for hydraulic plug movement will no long exist. 
During the transient period, before pressure balancing occurs, the plug 
member can be under the influence of a pressure induced resultant force. 
Plug movement toward its base extremity is not detrimental from the 
standpoint of plug locking because the plug is in effect being moved away 
from the apex taper of the valve chamber. Since the area of the plug 
member is greater at its base extremity A.sub.3 than the area A.sub.2 at 
the apex extremity thereof. A reduction in valve chamber pressure P.sub.1 
acting upon the area differential of the plug develops a resultant force 
urging the plug member towarde its apex extremity in conventional tapered 
plug valves. This occurrence can develop pressure induced plug locking. 
Such pressure induced plug locking is avoided in the present invention by 
the provision of a check valve to increase the rate of fluid flow from the 
base chamber to the flow passage. 
POSITIVE PRESSURE TRANSIENT 
During a pressure transient, the pressure increase in a chamber is governed 
by the volume "V" of the chamber and the respective clearance flow area 
"a" that allows the line pressure to equalize through the clearance flow 
area. Thus the pressure increase rate in any chamber is determined by the 
ratio a/V. 
In order to ensure that the apex chamber pressure increase rate is faster 
than the base chamber pressure increase rate, the following relationship 
should be maintained during a positive pressure transient: 
EQU a.sub.2 /V.sub.2 &gt;a.sub.3 /V.sub.3 
Therefore, 
EQU V.sub.3 &gt;V.sub.2 .times.(a.sub.3 /a.sub.2) Equation (1) 
The clearance flow areas a.sub.2 and a.sub.3 are fixed by the taper of the 
plug, the length, and the design clearance of the plug. For any given plug 
valve generally, the ratio a.sub.2 /a.sub.3 is fixed, and is proportional 
to the diameter of the large end and the small end. Since this ratio is 
fixed or cannot be changed, the relationship of Equation (1) can be 
achieved by proportionately making the lower chamber volume larger to 
satisfy the relationship set forth in Equation (1). 
When a pressure increase occurs in the flow passage chamber C.sub.1, the 
ball check 64 is maintained at its sealed position by the force of spring 
66 and by the force of pressure differential between line pressure and 
pressure within chamber C.sub.3. Pressure equalization between line 
pressure and the pressure of chambers C.sub.2 and C.sub.3 thus occurs 
through the annular clearance areas a.sub.2 and a.sub.3 between the 
tapered plug and the sealing surface 32. Since the base plug area A.sub.3 
is larger than the apex plug area A.sub.2, the result is a net downward 
acting pressure induced force on the plug member holding it securely 
against the adjustment screw. As pressure equalization takes place due to 
fluid transfer through the clearance between the plug member 28 and the 
tapered sealing surface 32 of the valve chamber, the apex chamber C.sub.2 
will reach pressure equilibrium before the base chamber C.sub.3 by virtue 
of the larger volume at the base portion of the plug. 
In the pressure/time curve of FIG. 4, line pressure P.sub.1 is steady 
during the initial time period A and then is increased to an elevated 
pressure level during time period B. At time point T.sub.1, pressures 
P.sub.2 and P.sub.3 within chambers C.sub.2 and C.sub.3 begin to increase 
with equalization flow occurring only in clearance areas a.sub.2 and 
a.sub.3. Pressure equalization is substantially complete at time point 
T.sub.3. The time/force curve of FIG. 5 illustrates downward load on the 
plug member 28 during the pressure chamber of FIG. 4. At time point 
T.sub.1, net downward force on the plug member is initiated due to 
pressure differential as pressures P.sub.2 and P.sub.3 equalize at 
different rates in chambers C.sub.2 and C.sub.3. The force is greatest 
where the pressure differential becomes greatest, as shown by the pressure 
differential arrow in FIG. 4. The net downward force on the plug then 
dissipates as the pressures in chambers C.sub.2 and C.sub.3 become 
equalized with line pressure. Thus, it can be seen that during the 
positive pressure transient, a downward force bias is maintained on the 
plug which prevents any plug movement toward the apex, thereby preventing 
tapered locking. 
NEGATIVE PRESSURE TRANSIENT 
During the negative pressure transient, if pressure equalization was again 
allowed to occur only through the clearance areas a.sub.2 and a.sub.3, it 
is obvious that the choice of V.sub.3 &gt;V.sub.2 .times.(a.sub.3 /a.sub.2) 
which was needed to maintain a faster pressure equalization rate between 
the top chamber and the line pressure, would now work against the design 
goal of keeping the plug force biased in the downward direction. The high 
pressure in the chambers V.sub.2 and V.sub.3 will start to decay during 
this negative transient, but it will decay at a slower rate in the lower 
chamber (as compared to the rate of decay in the upper chamber). 
Therefore, the plug would have experienced an upward force toward the 
apex, causing the plug to lock. 
In order for the plug to avoid locking during such negative pressure 
transients, another flow area has, therefore, been introduced according to 
the teachings of this invention. This additional flow area is represented 
by the cross-sectional area of a pressure balancing passage controlled by 
a check valve and designated "a.sub.check ". Therefore, the rate of 
pressure equalization during the negative pressure transient is governed 
by the ratio 
EQU (a.sub.3 +a.sub.check)/V.sub.3 Equation (2) 
for the lower chamber, and by the ratio 
EQU a.sub.2 /V.sub.2 Equation (3) 
for the upper chamber. 
In order to ensure that the pressure equalization rate is higher in the 
lower chamber during the negative pressure transient, it must be ensured 
that: 
EQU (a.sub.3 +a.sub.check)/V.sub.3 &gt;(a.sub.2 /V.sub.2) 
or 
EQU V.sub.3 &lt;V.sub.2 (a.sub.3 +a.sub.check)/a.sub.2 Equation (4) 
which can also be expressed as 
EQU a.sub.check &gt;(V.sub.3 /V.sub.2).times.a.sub.2 -a.sub.3 Equation (5) 
Equations (1) and (5) define the selection of V.sub.2, V.sub.3 and 
a.sub.check 
EQU V.sub.3 &gt;V.sub.2 (a.sub.3 /a.sub.2) 
and 
EQU a.sub.check &gt;(V.sub.3 /V.sub.2).times.a.sub.2 -a.sub.3 
These pressure and force relationships are evident from the correlated 
graphical representations of FIGS. 2 and 3. During time period A the 
pressure P.sub.1 is steady. At time point B a line pressure decrease 
begins and pressure again becomes steady at its lower pressure value. At 
point B fluid transfer from chambers C.sub.2 and C.sub.3 will begin as 
evidenced by pressure decay curves P.sub.2 and P.sub.3, with rates of 
pressure equalization differing according to the above equations. The 
pressure differential between pressures P.sub.2 of chamber C.sub.2 and 
P.sub.3 of chamber C.sub.3 will be at its greatest at time point D as 
evidenced by the pressure differential arrow. 
As is evident from FIG. 3, the downward force acting on the plug member 
during the negative pressure transient of FIG. 2 is shown in correlated 
manner. The downward force (acting toward the large end of the plug and 
considered in absence of spring preload force) begins at point T.sub.1 
when the pressure change occurs. This force increases to its maximum near 
point T.sub.2 and then gradually returns to zero as the pressures of 
chambers C.sub.2 and C.sub.3 become balanced with the lowered line 
pressure at T.sub.3. 
In the preferred embodiment disclosed herein the plug member 28 is formed 
to define a pressure equalization passage 60 defining flow passage 
a.sub.check and forming an internal valve seat 62 against which is seated 
a check valve 64. The check valve is maintained in seated relation with 
the valve seat 62 by means of a compression spring 66 which is secured 
within the passage 60 by means of a spring retainer element 68. As 
pressure P.sub.1 increases within the flow passage C.sub.1 of the valve 
such pressure will be prevented from free communication with chamber 
C.sub.3 by the check valve member 64. Flow passage pressure P.sub.1 acting 
upon areas A.sub.2 and A.sub.3 will, due to the area differential, develop 
a resultant force imparting downward bias to the plug member 28. Under 
conditions of negative pressure transient, when flow passage pressure 
P.sub.1 is reduced, pressure within the base chamber C.sub.3 of the valve 
will be quickly vented past the check valve 64 into the flow passage, 
causing chamber pressure P.sub.3 to quickly equalize with line pressure 
P.sub.1. During the transient period, pressure within the apex chamber 
C.sub.2 of the valve will act upon the pressure responsive area A.sub.2 
defined by the apex extremity 52 of the plug member thereby developing a 
pressure induced force acting upon the plug member 28 in a direction 
toward the base extremity thereof. During the transient period, pressure 
in chamber C.sub. 2 will continuously decay and the resultant force acting 
on the plug member will continuously diminish until it becomes 
substantially zero. 
Obviously, downward movement of the plug member 28 beyond the position 
shown in FIG. 1 cannot occur since the adjustment screw 34 provides a 
downward stop in the form of support surface 38 whether the plug member is 
urged downwardly by the compression spring system 54 or by the hydraulic 
influence of positive or negative hydraulic pressure transients. The net 
result is that the plug member will always remain in a properly adjusted 
position for optimum sealing. There will be no tendency for the plug 
member to be urged in the direction of its apex taper and thus no tendency 
for taper locking to occur. Even further, due to temperature transients, 
expansion and contraction of the valve chamber will not induce plug taper 
locking since the plug member will be urged by the compression spring 
system 54 to the position shown in FIG. 1. If the valve is positioned with 
the large end of the plug member located above the small end, the force 
spring 56-58 will prevent gravitational movement of the plug toward its 
apex taper. 
When tapered plug valves are utilized in gas service, the taper locking 
problem is particularly difficult to prevent. This is because of the 
absence of fluid in the sealing chambers eliminates the helpful effects of 
"fluid" spring or damping which would otherwise be available to restrain 
pressure induced plug movement. Furthermore, if the quantity of sealant or 
lubricant is diminished or absent altogether, the beneficial damping 
effects normally provided by the sealing and lubricating material will not 
be available. This further exacerbates the taper locking problem. Computer 
aided simulation has shown that under these worse-case circumstances, a 
spring preload is desirable to help prevent plug movement. In the present 
invention, this preload is provided by spring members 56 and 58 which 
simultaneously urges the plug member toward its base and urges the valve 
stem in the opposite direction against the inner surface 48 of the valve 
chamber. In combination with proper sizing of the apex and base chambers 
C.sub.2 and C.sub.3 and proper flow capacity of the ball check valve 
mechanism, the magnitude of spring preload can be minimized, thereby 
maintaining valve operating torque at acceptably low levels. A further 
beneficial effect of the spring preload is to prevent taper locking due to 
thermal transients or to prevent gravitational movement of the plug member 
toward its apex taper due to its own weight when the valve is installed 
with the base portion of the plug facing upwardly. 
In view of the foregoing, it is respectfully submitted that a non-locking 
tapered plug valve mechanism has been provided herewith which accomplishes 
all of the features and objects hereinabove said forth together with other 
features which are inherent in the valve mechanism itself. It will be 
understood that certain combinations and subcombinations of this invention 
are of utility and may be employed without reference to other features and 
subcombinations. This is contemplated by and is within the scope of the 
present invention.