"Valve Signature Basics"

Let us cover the basics and understand just what the diagnostic traces should look like on a good performing valve. This feature will be dedicated to looking at good signature curves and explaining what to expect from both the data trace and the auto analysis.

In this feature we will be looking at the "Valve Signature" plot. This plot shows the integrity of the valve body and actuator assemblies. The plotting scheme is a bit different then what we are accustomed to in that the input (Actuator Net Pressure) is plotted on the "Y" axis while the output (Travel) is plotted along the "X" axis. This was done intentionally in order to show changes in force. By plotting the data in this fashion, any increase or decrease in force would be shown as a vertical change on the graph.

What does one look for on a typical "Valve Signature" plot? First you must see spikes (change of slope) at each end of the curve. This verifies that a solid stop had been reached at both ends of travel. This is very important, for without a visual indication that the stops were reached, one could not verify that full travel had been achieved, nor that the analyzed benchset is correct (Fisher uses the top actuator stop as the reference point when establishing benchset).

Next, look to see that the opening (red) and closing (blue) lines are parallel and linear throughout the full stroke. The separation of these lines is the result of the friction band. The higher the friction the wider the separation. Because friction always apposes motion, the net spread of these lines is actually double the friction (friction apposing the up stroke plus friction apposing the down stroke). The primary source of friction on a good valve is the valve packing. Packing materials that have a high coefficient of friction, such as graphite, will produce a greater amount of friction and thus a wider bandwidth then the low coefficient materials, such as PTFE. One would expect the friction band (line spread) to remain constant throughout the full travel regardless of the packing material used.

Also in viewing the opening/closing lines of the graph above we see a slope in the data. This slope indicates that the actuator contains a spring. If there were no spring, the opening and closing lines would be nearly flat (horizontal). The actuator spring (spring rate) and the actuator size (effective area) govern the slope’s angle.

Once you’ve made a mental note as to the appearance of the signature curve, it is now time to allow the computer to analyze the data. You should have noticed that there was an additional green line drawn on the signature curve. This line represents the best-fit line between all the data points. It is from this best-fit line that much of the analysis is being derived.

Let us first look at friction. Here we see Minimum, Maximum, and Average Friction values. These values are derived by looking at individual pairings of adjacent upstroke/down stroke data points between 10-90% of the travel range with ValveLink or 5-95% with FlowScanner. The Minimum Friction value should never be less then 25% of the expected friction value (20% if PTFE packing). The Maximum Value should never exceed 100% of the expected value. Nor should there be a large difference between the Minimum and Maximum values. The Average friction value will typically be 40-60% of the expected value. The equation for determining the frictional force values is essentially Ff = (?Pact X Aeff) / 2 or, in other words, the difference in actuator pressure between the upstroke and down stroke, times the effective area of the actuator, divided by two.

Now we enter the field called Benchset. Benchset per Fisher definition is the windup of the actuator spring to achieve a mechanical force equal to or greater then all the forces acting against the valve throughout its rated travel range while in service. Those forces being process forces, frictional forces, seating force, and forces do to special assemblies (such as those with multiple piston rings). Benchset is a predetermined value that is established during the actuator sizing procedure. Actuator size, spring rate, and rated valve travel all influence the benchset span. Setting this spring windup is done with the actuator disconnected from the valve so as not to introduce any forces. Frictional forces in particular. This procedure is done during the actuator assembly procedure while the actuator is lying on the workbench far removed from the valve it is to be installed on. Hence the terminology "Benchset". Typically valves utilize two stops. One being the valve seat and the other the top actuator stop. Now with the actuator removed from the valve, there is no way of knowing where the bottom stop (seat) will be. Thus, the only common reference point for establishing Benchset would be the upper stop of the actuator. With this in mind, the Benchset spring windup and span is established from the upper actuator stop down to the rated travel of the valve on which the actuator will be installed. This is true for both direct and indirect actuators. The computer analysis determines Benchset along the best-fit line for this represents the valve travel null of friction. It looks for the last travel reading along the best-fit line at the open end and records the pressure at that point. It then extends down the slope of this line until rated (not actual) travel is reached and records the pressure at that point. The two pressure readings will be the measured Benchset.

Next we see "Seat Load as Tested" and "Service Seat load". Seat Load as Tested is irreverent at this time for it is used mainly as a means for establishing Service Seat Load. The Service Seat Load is the important guy and its value must be equal to or greater then the specified seat load in order to meet the valve’s shutoff classification. Seat load is the additional force available from the actuator after seat contact has been made. Now the present diagnostic software (January 2001) cannot distinguish the actual contact point and thus assumes seat contact has been made at 0.015" of travel for valves traveling 1" or less and 0.035" for travels greater then 1". This is the procedure set forth in ValveLink. FlowScanner picks the point at 1.5% of total travel regardless of travel range. The Seat Load as Tested value is determined by multiplying the delta actuator pressure times the effective actuator area. Delta actuator pressure is the difference between the last recorded pressure reading at the closed end and the pressure at the seat contact point (defined above). Service Seat Load is a calculated value that takes into consideration the effect of process pressures on the unbalance area of the valve. The maximum expected process pressure values (P1 and P2) at shutoff must be entered into the Spec Sheet (ValveLink) or Nametag (FlowScanner) in order to analyze Service Seat Load.

Now that we are satisfied with the valve signature trace and data analysis, there is one more thing that we must view before exiting, that being the seating profile. The seating profile is defined by zooming in on the area of the curve near the seat.

<< Back to Dabbling in Diagnostics Index