Part I: Conventional Flow Directionfor Globe Control Valves

Globe control valves are generally dividedinto two structural types:

- Unbalanced type (also calledtop-guided)

- Balanced type (also calledcage-guided)

Flow direction is also generally classifiedinto two types (this definition is very clear for unbalanced plugs; for balanced plugs, it still needs discussion—let's setthat aside for now and usethe following descriptions for the moment):

- FTO (Flow to Open, F-T-O): Medium flows infrom below the plug, pushing it upward to open

- FTC (Flow to Close, F-T-C): Medium flows infrom above the plug, pushing it downward to close

The diagram makes it clearer:

**FTO** **FTC**
General flow direction selectionprinciples are as follows:

- Unbalanced plug → FTO flow direction

- Balanced plug (liquid medium) → FTC flow direction

- Balanced plug (gas or steam medium) → FTO flow direction

For unbalanced plugs, FTO is chosen becauseusing FTC would create a "piston effect" at small openings, causing plug instability.
However, for on/off valves or regulatingvalves that don't operate at small openings, FTC is actually not bad at all! On
the contrary, it's beneficial for sealing and provides a larger Cv value.

For balanced plugs, the reason for choosing FTC for liquid media is: since the cage windows are symmetrically distributed around the cage, liquid passing through impacts and cancels out energy. The reason for choosing FTO for gas media is: flowing from inside the cage throughthe windows into the larger valve body cavity facilitates pressure drop.

All of the above are theoretical rulesrecommended in textbooks or vendor literature for instrumentation engineers
during selection. However, for special operating conditions, these theoretical analyses may not be correct! These "rules" warrant further discussion.

Part II: Cage Valve Flow Field Simulation

This section presents conclusions first,then analyzes the reasons.

Below is our flow field simulation analysisfor an FTO cage valve: upstream pressure 3 MPa, downstream atmospheric pressure, air medium, 20°C.

The same analysis was conducted for FTC.The diagram isn't shown here; instead, the data is presented directly below
(units in Newtons):

Note:
- X-axis force is in the fluid flowdirection: Positive in the same direction as fluid flow, negative when opposite

- Y-axis force is in the actuatordirection: Upward is positive, downward is negative

Both Fx and Fy exist, and theirconsequences differ. Fx is the cause of eccentric wear between the plug and seat, while Fy is the direct cause of unstable valve position.

Part III: Detailed Analysis of Fy

From a static perspective—or when the valve is closed without flow—thepressures in the upper and lower chambers of a balanced plug are equalized through balance holes. However, once medium flows through, the flow field becomes dynamic.

The above diagram shows an FTC balancedplug. Section A has flow velocity, especially near the throttling port at C where velocity is extremely high. According to Bernoulli's equation, pressure at section A will be lower than at static section B, and pressure at C will be even lower. Therefore, in flow field analysis, Fy is negative, indicating the plug experiences a downward force. The conclusion is that the flow field-induced unbalanced force acts downward.
What guidance does this analysis providefor FTC vs. FTO selection? Let's illustrate with actual calculations:

Assume a BAC anti-surge valve recirculatingfrom 30 bar to 5 bar:

→ FTO Scenario:

When closed, PB, PA, and PC are all 30 bar.When opened, pressure at C drops most significantly due to high velocity, while PA is slightly lower than PB. The pressure cloud diagram below confirms this conclusion:

Thus, a pressure differential is createdbetween PB and PC. This differential pressure multiplied by the plug's annular
area approximately equals Fy.

Assuming PA = PB = 30 bar, PC = 22 bar(estimated from pressure cloud), the differential pressure between PB and PC is 8 bar.

→ FTC Scenario:

When closed, PB, PA, and PC are all 5 bar.When opened, pressure at C drops most, PB ≈ 5 bar, andthe average pressure at C must be lower than 5 bar. Therefore, the differential
pressure between PB and PC must be less than 5 bar.
All data above are assumptions, but it's clear that Fy in FTC is smaller than in FTO. Comparing this with the flow field analysis values makes it very obvious.

The conclusion: For balanced plugs, regardless of low-in/high-out or high-in/low-out (referring to valve body, not plug), the medium flow-generated force tends to close the plug. So shouldn't they all be called FTC? Or to avoid confusion, should FTC and FTO be redefined for balanced valves?

Practical case (170,000 Nm³/h, 34 bar BAC): Severe vibration and unstable valve position werecaused by Fy-direction unbalanced forces (flow fields are never perfectly stable). Valve specifiers may have a misconception: with balanced structures, medium differential pressure doesn't act on the plug, so actuator thrust only needs to provide sealing pressure and packing friction. Consequently, they select smaller actuators with lower spring coefficients (K). Any slight flow field instability causes spring oscillation, and positioners simply cannot keep up with this oscillation speed, making it impossible to converge. The solution requires increasing the spring coefficient (K)—in otherwords, increasing actuator thrust.

Part IV: Detailed Analysis of Fx

Actually, Fx is easy to understand:differential pressure before and after throttling pushes the plug downstream, causing eccentric wear. Of course, we need data to support this.

FTO Scenario:

After passing through the throttling port,fluid expands in all directions, but conditions at sides A and B are completely different. Fluid at side A impacts the valve body, creating a reaction force that pushes the plug:

The pressure cloud clearly shows differentpressures at sides A and B. This pressure differential creates a thrust on the plug, causing eccentric wear.

FTC Scenario:

After passing through the throttling port,the structure is completely symmetric around the plug and seat, so pressure differences at various points are minimal. While the upstream flow field isn't perfectly uniform either, it's significantly better than FTO due to lower upstream velocities.

HP GOX Vent Valve (FTO) Wear

*MAC Vent Valve (FTO)*

Part V: Another Case of FTO-Induced Problems:

A 67,500 Nm³/h MACunit: After startup, the vent valve was gradually closed to send gas to the process. However, the vent valve could never fully close (approximately 3% remaining). Based on oxygen production rates, leakage was about 2,000 Nm³/h of air. The ASU had been operating for over two years with nosolution. Interestingly, if the vent valve was opened before shutdown, then closed after shutdown, it could seal properly. Later, during a shutdown, the valve was disassembled:

Two problems were identified:
1. Metal strips at the valve bottom:During valve opening, due to pressure differential across the perforated cage, metal strips were sucked against the cage inner wall. When the plug closed downward, the strips jammed between the plug and seat. However, after shutdown with no flow, the strips fell back to the valve bottom by gravity. With FTC, foreign objects would be blocked by the cage and couldn't enter the sealing surface.

2. Eccentric wear between cage andplug.

In Conclusion

The evidence from both flow simulation andfield performance is unequivocal: conventional flow direction "rules" for compressor anti-surge valves frequently lead to suboptimal outcomes. For balanced globe valves, FTC configuration consistently proves superior—minimizing dynamic forces that cause instability and wear while providing superior tolerance to real-world conditions like pipeline debris. The cases we've documented aren't isolated exceptions; they're cautionary tales that underscore why theoretical guidelines must be validated against actual flow mechanics.

Your experiences matter: We welcome field data and discussions that challenge or corroboratethese findings. After all, advancing valve technology requires collective wisdom from those who live with these installations daily.