[NEFSS Tech Series #05] PSV Core Settings and Terminology: From Set Pressure to CDTP
- nick so
- Feb 2
- 9 min read
Hello, I'm Nick from NEFSS.
In the previous session, we addressed practical external specifications such as Face-to-Face (F-to-F) dimensions and accessory selection, which resonated with many engineers. It was a time dedicated to resolving those on-site frustrations, such as "The size is correct, so why won't it install?"
We have finally reached the final installment of our series. In this episode, we will summarize how PSVs are set during the manufacturing stage and how those setting values relate to actual field operating conditions. We will also clarify key technical terms that frequently appear in catalogs but may feel ambiguous. Understanding the difference between Set Pressure and Closing Pressure, as well as how factors like Stellite treatment and CDTP affect PSV performance and reliability, will allow you to view the PSV not just as "equipment to be installed," but as a safety device designed to operate precisely under intended conditions.
In this final part of the series, we will calmly summarize the core information you must know from a field application perspective.
Procedures for Adjusting Safety Valve SET PRESSURE and CLOSING PRESSURE
1) How to Adjust SET PRESSURE
To adjust the set pressure, first open the CAP, loosen the LOCK NUT, and
use the Adjusting Screw (Adjustment Bolt) to make the modification.
*Turn Clockwise: The adjusting screw moves downward, further compressing
the spring. → This increases the SET PRESSURE.
*Turn Counter-clockwise: The adjusting screw moves upward, reducing the spring tension. → This decreases the SET PRESSURE.
After operating the valve multiple times to verify that it functions accurately at the intended pressure, secure the set pressure with the LOCK NUT. Finally, close the cap and perform the SEALING.
(Red arrow: LOCK NUT // Blue arrow: Adjusting Screw)
Procedures for Adjusting Safety Valve Set Pressure (by NEFSS)
2) How to Adjust CLOSING PRESSURE (= How to Adjust BLOWDOWN)
The CLOSING PRESSURE is adjusted using the ADJUST RING (Upper Ring /
Lower Ring). The closing pressure is fine-tuned by rotating the
gear-shaped ADJUST RING shown in the photo. It is easy to understand if you
think of it like adjusting the "clicks" on a rifle sight when your aim is off
during military marksmanship training—you rotate it bit by bit until you
reach the desired CLOSING PRESSURE. Once the adjustment is complete,
secure the ADJUST RING with a SET SCREW (a sharp fixing screw) to prevent it
from moving, and then perform the sealing.
3) Importance of Sealing
Since SET PRESSURE and BLOWDOWN are critical factors directly related to the performance of a safety valve, any unauthorized adjustment can lead to severe safety issues. Therefore, immediately after the accurate setting and verification are completed by the manufacturer or an authorized testing facility, Sealing must be performed to prevent unauthorized tampering.
Occasionally, seals are arbitrarily broken and units disassembled on-site under the pretext of "product defects." However, in such cases, it becomes difficult to conduct an accurate root cause analysis, and each manufacturer (MAKER) may refuse to honor the warranty.
👉 Arbitrary disassembly and the act of damaging seals must be strictly avoided.
BLOWDOWN
1) What is BLOWDOWN?
It refers to the difference between the SET PRESSURE and
the CLOSING PRESSURE,and is generally expressed as a percentage (%).
Example
SET PRESSURE: 10 barg
CLOSING PRESSURE: 9 barg → In this case, it is commonly described as having a "10% BLOWDOWN."
While the allowable standards differ slightly for each CODE, they are
generally understood as follows:
KOSHA: Within 10%
KGS: Within 15%
(*Note: As this varies depending on the pressure, it is essential to check the specific standards of the relevant CODE.)
2) Why is BLOWDOWN Important?
Imagine a situation in the middle of winter where your house is nice and warm,
but the door opens and cold air starts rushing in.
If the first person closes the door in 3 seconds, but the second person takes
10 seconds to close it, the longer the door stays open, the more warm
air escapes.This results in significant energy loss as the boiler must work harder
to reheat the house.
Safety valves work the same way. The longer it takes for the valve to close after
activation, the greater the energy loss; therefore, users typically prefer the valve
to close as quickly as possible. However, from a manufacturer's perspective,
setting the BLOWDOWN too small can lead to unstable valve operation.
For this reason, it is set within an optimal range in accordance with relevant
CODEs and licensing standards.
What is STELLITED?
This refers to the process of melting and applying a special alloy (overlay welding)
to the area of the valve under the most stress—the contact surface where the
Seat and Disc meet. This reinforces the contact surfaces and is also known as
Hard Facing.
When reviewing a PSV DATA SHEET, you may see the Seat and Disc materials
marked as "ST." This should be understood to mean Stellited (e.g., A276 316 + ST.).
As a side note, parts treated with Stellite are extremely precise and sensitive
components. If scratches occur on the seat surface due to fine foreign substances
in the fluid or sludge within the piping, it can lead to immediate leakage.
Therefore, we urge you to handle these products with extreme care during
transportation, installation, and especially during initial line flushing.
OVERPRESSURE
If you look closely at a PSV Data Sheet, you will see figures like 10% or 21%
specified under the 'Overpressure' item. This is a crucial value determined by
the Sizing Basis or Cause of Overpressure (calculation basis) scenario.
1) Overpressure Scenarios and Applied Values
There are various reasons why pressure rises within a process. Representative
examples include a blocked outlet at the downstream of a valve (Blocked Outlet),
expansion due to temperature rise (Thermal Expansion), control valve failure
(CV Failure), and external fire (External Fire).
The core standards to remember are as follows:
Fire Case (21%): If the word "Fire" is included in the scenario, an overpressure
value of 21% is applied.
Non-Fire Case (10%): For general process abnormalities other than fire,
you can understand that 10% is usually applied.
2) When is the 16% value used?
Occasionally, you may see the number 16%, which is used for
Multiple Valve specifications. When the required discharge capacity is too large
for a single safety valve to handle, two or more valves are installed in parallel.
This 16% is the standard applied to the first valve in such cases.
(Since this is not a common case, it is sufficient to know it as a standard used
for multiple valve installations.)
3) What is the true meaning of Overpressure?
To explain it simply, it is the allowable range of "how high the pressure can rise
at maximum even after the safety valve has popped."
A safety valve begins to operate at the Set Pressure, but the pressure does not
drop immediately; instead, it continues to rise to a certain point while
being discharged.
For example (Based on Set Pressure 10 barg / Overpressure 10%):
The fact that the PSV activated at 10 barg does not mean the pressure
drops instantly. It means the pressure can continue to rise through 10.3 and
10.5,reaching a maximum of 11 barg (10% above the set pressure).
In other words, the system is designed to allow a rise up to 11 barg but ensures
it does not exceed that limit.
What about a Fire Case (21%)?
With the same 10 barg setting, it means the pressure can rise up to a maximum
of 12.1 barg during a fire situation.
CDTP (=SPRING SETTING PRESSURE)
In PSV Data Sheets, you will frequently encounter the term CDTP.
According to the API 520 Code, this stands for Cold Differential Test Pressure.
Simply put, it refers to the value used to compensate for the
environmental differences (such as temperature and pressure) between the
factory where the safety valve is manufactured and tested and the actual site
where it is installed.
For instance, even if the field conditions are harsh—with temperatures
exceeding 150°C and constant back pressure—it is difficult to replicate those
exact conditions at the manufacturing plant. Therefore, the valve is set to
a pre-compensated pressure during the factory release stage to ensure
it operates accurately at the intended pressure on-site.
CDTP compensation primarily consists of two factors:
Pressure Compensation and Temperature Compensation.
1) Pressure Compensation
Pressure compensation is applied to Conventional Type valves
when Constant Back Pressure exists.
Impact of Constant Back Pressure: "Constant Back Pressure" is the pressure always present at the valve outlet. Since this back pressure acts in conjunction with the spring force pushing down on the disc, it may prevent the valve from operating at the designated pressure.
Compensation Method: If set at the factory without compensation, the field operating pressure would need to exceed the Set Pressure for the valve to pop, as the back pressure adds to the spring force. Therefore, to ensure accurate operation on-site, the valve must be set at the factory to
[Set Pressure - Constant Back Pressure].
Note: Bellows Type valves are structurally designed to be unaffected by back pressure. Thus, separate pressure compensation is not required in this case.
2) Temperature Compensation
As temperature rises, the elasticity of the metal spring weakens.
To ensure the valve operates accurately at the designated pressure under
high-temperature field conditions, this must be accounted for when setting
the valve at the room-temperature factory.
While temperature compensation coefficients may vary by manufacturer (Maker),
below is a general guideline for reference.
(※ The figures below are examples for illustrative purposes only.)
Operating Temp. | Compensation Factors (Example) |
Below or equal to 121°C | 1.00 (No Compensation) |
Above 121°C ~ Below or equal to 315°C | 1.01 |
Above 315°C ~ Below or equal to 426°C | 1.02 |
Above 426°C ~ Below or equal to 537°C | 1.03 |
The compensation procedure must be performed in a specific order:
first, carry out [Step 1: Pressure Compensation], and then apply
[Step 2: Temperature Compensation] to that resulting value.
The final value derived from this process becomes the CDTP for that specific PSV.
3) Examples of CDTP Calculation by Case
Let’s look at a few examples to see how the actual CDTP is calculated.
Example 1) Conventional Type (High Temp. + Constant Back Pressure)
Conditions: Set 10 barg / Constant Back Pressure 0.5 barg / Temp. 150℃
① Pressure Compensation: 10 - 0.5 = 9.5 barg
② Temperature Compensation: 9.5 barg x 1.01 = 9.595 barg(Final CDTP)
Example 2) Conventional Type (Low Temp. + Constant Back Pressure)
Conditions: Set 10 barg / Constant Back Pressure 0.5 barg / Temp. 40℃
① Pressure Compensation: 10 - 0.5 = 9.5 barg
② Temperature Compensation: No compensation (Factor 1) = 9.5 barg (Final CDTP)
Example 3) Bellows Type (High Temp. + Constant Back Pressure)
Conditions: Set 10 barg / Constant Back Pressure 0.5 barg / Temp. 150℃
① Pressure Compensation: Not applied (Bellows Type does not require separate pressure compensation) = 10 barg
② Temperature Compensation: 10 barg x 1.01 = 10.1 barg (Final CDTP)
4) Conclusion: Why is CDTP Setting Essential?
Even if the set pressure requested by the client is 10 barg, the PSV must be
shipped only after being set according to the CDTP value.
If the PSV is shipped simply set to the Set Pressure of 10 barg,
there is a possibility that it will not operate normally at the intended pressure
because the actual field pressure and temperature conditions were not
sufficiently reflected.
Therefore, to ensure that the PSV operates accurately under field
operating conditions, it is essential to set and ship it based on the CDTP,
which accounts for both pressure and temperature factors.
For this reason, CDTP is not just a theoretical calculation but is referred to
as the actual "Spring Setting Pressure" configured at the factory.
🛠️Technical Consultation & Request for Quotation
If you are having difficulty selecting the right safety valve or need a fast and reliable quote, please feel free to contact NEFSS at any time!
[Closing the PSV Technical Series: Gratitude and a New Promise]
This material has been prepared with the hope of providing practical help to everyone in the field—from plant operators and manufacturing/supply chain professionals to our valued clients and new employees just starting their careers. I sincerely hope these records serve as a helpful guide in understanding the practical aspects of safety valves.
Above all, this is my personal "Final Report," carefully compiled to express my gratitude to the many business partners I have communicated closely with as I move on to a new beginning. Although my affiliation and position have changed, my desire for this material to be a small contribution to your work remains the same.
With this, I conclude the 5-part PSV (Safety Valve) technical series. Since this material focuses on basic concepts and practical applications, I recommend consulting the manuals and official technical data of each manufacturer for precise and detailed technical specifications, and seeking advice from manufacturer engineers when necessary.
While the safety valve series ends here, my journey does not stop. I promise to continue introducing more in-depth and useful information through NEFSS, covering not only safety valves but also various other types of valves and piping materials essential for plant sites.
I extend my deepest respect to all of you who strive for safety and quality in the field every day. Thank you sincerely for reading this series until the end. I wish you continued health and peace in all your future endeavors.
At NEFSS, we go beyond simple material supply to become a partner that cares about your site's safety.
Thank you. Best regards, Nick, NEFSS
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