SPECIAL-TASK VALVES PART 1

I'm special (special)So special (special)I gotta have some of your attention, give it to me

("Brass in Pocket," by the Pretenders, 1979)

SPECIAL-TASK VALVES
HAVE SPECIAL JOBS TO DO

Brilliant PTOA Readers and Students ...meaning those who are reading the PTOA Segments in the intended, sequential order ... just finished learning about the four jobs Valves perform in the Piping Network of a processing facility.

There are a few Special-Task Valves that perform special tasks.

Special-Task Valves include:

Left: The ISA Symbol for a Pressure Reducing/Regulating Valve. Middle: The ISA Symbol for a Back Pressure Valve. Right: The ISA Symbol for a Pressure Relief Valve.

Pressure Reducing/ Regulating Valves.
Back Pressure Regulating Valves.
Blow-Off/ Blow-Down Valves,
On-Off Valves and their Actuators
- Solenoids.
- Motorized Operated Valves.
- Hydraulic Operated Valves.

This PTOA Segment will focus on the form, function, and confusing nomenclature of Pressure Reducing/Regulating Valves and Back Pressure Regulating Valves.

The Pressure Reducing/Regulating Valve performs a similar task to the Step-down transformers shown in this graphic. A Step-down transformer decreases the high 400kV voltage to a much lower 13kV voltage which is further stepped down to 240V ...a lower voltage which is safely usable in the house. Likewise, a Pressure Reducing/ Regulating Valve decreases the high PV Pressure of a distributed process fluid to a safe flowing pressure that can be safely used.

These two types of valves are piped directly into the flowing path of the process stream which enables them to simultaneously impact the steadiness of the PV Flowrate and the magnitude of the flowing PV Pressure.

However, before delving into the form and function of the hardware some attention must be paid to learning the terms "Upstream" and "Downstream."

Understanding these terms is a prerequisite to competently understanding the function of Pressure Reducing/ Regulating Valves and Back Pressure Regulating Valves.

THE MEANING OF "UPSTREAM" and "DOWNSTREAM" IN A PIPING NETWORK

The descriptors "Upstream" and "Downstream" describe the relative position of a fluid flowing through The Piping Network.

There is a hardware component in the middle of this pipe. The YELLOW stream has a black arrow which indicates the flow direction is to the right. The YELLOW stream is flowing UPSTREAM of the hardware component. The GREEN stream is flowing DOWNSTREAM of the hardware component.

First, pick a reference point within the Piping Network ... like a Valve or a Pipe Fitting.

The fluid flowing into the Valve is "Upstream" and the fluid flowing out of the Valve is "Downstream."

Otherwise stated:

PTOA Readers and Students will recognize this graphic of the Typical Centrifugal Pump Piping Setup. The Tank, Inlet Gate Valve, and Reducer Fitting are all UPSTREAM of the Pump. The Expander Fitting, Check Valve, and Discharge Gate Valve are all DOWNTREAM of the Pump.

Upstream = physically positioned at the inlet side of a piece of hardware in the Piping Network. The term can be applied to meaning directly in front of the hardware or at a distance away, flowing toward the hardware.

Downstream = physically positioned at the outlet side of a piece of hardware in the Piping Network. Ditto regarding the term being used to mean directly at the outlet of the hardware or flowing some distance away from the hardware.

Focus on the Reactor. Can you decode that the (Combined Hydrogen and Liquid) Feed stream, the heated process stream exiting the left side of the Combined Feed Exchanger (CFE), and the process stream flowing through the Fired Heater are all UPSTREAM of the Reactor? Likewise, the Reactor Effluent, the cooled fluid exiting the right side of the CFE, the (Reactor) Effluent Cooler, and the HP Separator are all DOWNSTREAM of the Reactor.

PTOA Readers and Students will soon learn that the flowing PV Pressure of the fluid that flows out of a Pressure Reducing/ Regulating Valve ... aka the downstream flowing PV Pressure ... provides the Setpoint Pressure for the Valve.

PTOA Readers and Students will soon learn that the flowing PV Pressure of the fluid that flows into a Back Pressure Valve ... aka the upstream flowing PV Pressure ... provides the Setpoint Pressure for the Valve.

SPECIAL-TASK VALVES THAT MANAGE
THE PV FLOWRATE & THE FLOWING FLUID'S PRESSURE

Pressure Reducing/Regulating Valves

Pressure Regulating Valves in natural gas service.

Pressure REDUCING Valves are often abbreviated PRV.

Because the Pressure Relief Valve that was just featured in PTOA Segment #255 is also abbreviated PRV, Process Operators must use mental awareness not to confuse these valves!

Fred must use his brain when he hears the spoken abbreviation "PRV" to figure out if the speaker is referring to a Pressure Reducing Valve, a Pressure Regulating Valve, or a Pressure Relief Valve!

Nor would life be complete without additional PRV-nomenclature confusion:

So, Fred ...

When the flowing fluid is a gas, the Pressure Reducing Valve is often referred to as a Pressure REGULATING Valve.

Many industrial professionals assume a Pressure Reducing Valve is used in a liquid process service whereas a Pressure Regulating Valve is used in a gas process service.

How to Distinguish a Pressure REDUCING/REGULATING Valve
From a Pressure RELIEF Valve

Pressure REDUCING/ REGULATING Valves and Pressure RELIEF Valves can be distinguished by:

ISA Symbol and
Job Description.

Decoding a Pressure REDUCING/ REGULATING Valve from a Pressure Relief Valve on a Process Flow Diagram (PFD) or Piping and Instrumentation Diagram (P&ID) is easy because their ISA Symbols are significantly different.

The ISA symbol for a Pressure Reducing/Regulating Valve will not be mistaken for a Pressure Relief Valve or Pressure Safety Valve. The angled-then-downward appendage is a hint that the Valve's Setpoint Pressure is the DOWSTREAM Pressure. Beware! The ISA Symbol for a Back Pressure Regulating Valve is easier to confuse with the ISA Symbol for a Pressure Reducing/Regulating Valve!

The nearby graphic depicts the ISA Symbol for a Pressure Reducing/ Regulating Valve.

The "angled-then-slanting downward" appendage that is attached to the top of the valve and extends to the downstream side of the valve body hints at the job the Pressure Reducing/ Regulating Valve performs.

This ISA Symbol is significantly different than the ISA Symbol for the Pressure Relief Valve shown in the vertically second position on the nearby chart labelled "Relief Valve Symbols."

Pressure Relief Valves and Pressure Safety Valves have different ISA symbols than Pressure Reducing/Regulating Valves. They are also physically accessed via a service that extends from the main process flow path.

Likewise, the job descriptions between a Pressure Reducing/ Regulating Valve and a Pressure Relief Valve are so distinct that these two types of Valves are physically positioned differently in the processing plant's Piping Network as well as on the PFD and P&ID.

The inlet to a Pressure Relief Valve is via a short process service line that branches off from the main process fluid flow path.

However ...

The Pressure Reducing/ Regulating Valve is piped directly into the flow path of the flowing fluid.

Here's an example of how easy it is to mess up "PRV" nomenclature: The encircled valve should be labelled Pressure Reducing Valve (not Pressure Relief Valve). This Pressure Reducing Valve is positioned between the (unseen) upstream higher-pressure source of cold water (perhaps the city's distributed water main) and the hot water Tank in a household. The job of the (mislabeled) Pressure Reducing Valve is to reduce the flowing water Pressure from the high-Pressure main source to a lower Set Point pressure which is determined by sensing the downstream flowing Pressure. During normal operations, the water flowing out of the (mislabeled) Pressure Reducing Valve would flow into the Water Tank to be heated. As shown in this schematic, an over-pressuring of the Water Tank has occurred thus the flow path is FROM the Tank Inlet to the Pressure Relief Valve that is located on the Pressure Relief Line. One good thing about this graphic is that it depicts how the Pressure Reducing Valve is piped directly into the normal flow path whereas the Pressure Relief Line and Valve branch off from the normally flowing path.

PTOA Readers and Students just learned in PTOA Segment #255 that the job of the Pressure Relief Valve is to manage the occasional, non-threatening over-pressuring situation which would otherwise gradually degrade Piping Network hardware.

To perform its job successfully, the Pressure Relief Valve will be physically located on a service process line, not on the main process flow path.

In contrast, the Pressure Reducing/ Regulating Valve is piped directly into the main PV Flowrate path so that the valve can continuously perform its job 24 hours a day, 7 days a week!

What is the job of the Pressure Reducing/ Regulating Valve?

Imagine a fluid flowing through a process line, fluctuating in flowing magnitude and at a higher PV Pressure than is desired downstream.

The job of the Pressure Reducing/ Regulating Valve is to reduce the higher inlet PV Pressure to a lower PV Pressure Set Point which is sensed directly downstream of the Valve's outlet.

The fluid exiting the Pressure Reducing/ Regulating Valve will no longer be fluctuating. The flow path will be steady and at a lower PV Pressure than the Valve's inlet PV Pressure.

The PTOA Department of Redundancy Department repeats:

The job of the Pressure Reducing/ Regulating Valve is to convert an upstream fluctuating flowrate that has a higher-than-desired PV Pressure into a steady outflowing process stream which flows at the desired lower PV Pressure.

These Pressure Regulators in natural gas service decrease the high flowing PV Pressure of the distributed source to a desired lower flowing PV Pressure suitable to the end user.

The location of a Pressure Reducing/ Regulating Valve in The Piping Network of a processing facility will always be away from where the fluid is originally generated and closer to where the fluid is distributed to an end user.

For example:

The flowing PV Pressure of the natural gas supplied to a Gas Turbine (aka, the Turbine's Fuel Gas) will need to be stepped down from the header Pressure to a Pressure which can be safely used by the Gas Turbine.

The flowing PV Pressure of the instrument air used in pneumatic Automatic Final Control Valves will need to be stepped down from the header supply pressure.

The job description described above for the Pressure Reducing/ Regulating Valves requires that the Pressure Reducing/ Regulating Valve be piped directly into the main process flow path.

Many thanks to Kimray.com for use of this photo and graphic of a Pressure Regulating Valve for gas service. The operation of a Pressure Regulating Valve involves Force-Balancing around the black Diaphragm. In this graphic: Red = Upstream gas high pressure. Blue = Downstream flowing gas Setpoint Pressure. Yellow = Pressure sensed by the interfacing Diaphragm which determines downstream flowing Pressure.

How does the Pressure Reducing/ Regulating Valve do its job?

Understanding the operation of a Pressure Reducing/ Regulating Valve starts with applying the concept of force balancing (featured in PTOA Segment #254) to the top and bottom of Diaphragm Valves which were featured in PTOA Segment #253. PTOA Readers and Students who desire more in-depth operational understanding of Pressure Reducing/ Regulating Valves should access the Kimray.com website.

What all Process Operators must be able to do is:

Identify/distinguish the Pressure Reducing/ Regulating Valve on a P&ID from other types of valves.
Understand what the downstream Setpoint Pressure is and why that PV Pressure has been selected for the process service.

Many thanks to Kimray.com for use of this graphic which shows how the Pressure Reducing/Regulating Valve (upper left quadrant) interacts with the Automatic Control Valve. This valve-combo is responsible for reducing the downstream Pressure to a Setpoint which is determined by sensing the downstream Pressure.

Very high-pressure processes ... which are the most common application in process industry ... require a Pressure Regulating Valve as a separately operating system that engages with an Automatic Control Valve.

In this type of service application, the Pressure Regulating Valve will determine how much an Automatic Control Valve will open or close.

The form and function of the Automatic Control Valve will be the focus of future PTOA Segments.

BACK PRESSURE (REGULATOR) VALVES

The ISA Symbol for a Back Pressure Regulator infers that the sense line is on the UPSTREAM side of the Valve. As shown, the sense line is internal to the Valve. Some Back Pressure Regulators have external sense lines that would sense the directly UPSTREAM process line Pressure.

The only way not to confuse the ISA Symbol for a Pressure Reducing/ Regulating Valve and a Back Pressure (Regulator) Valve is to be knowledgeable about their distinct job descriptions.

The descriptor in parentheses ... "Regulator" ... infers this type of Valve is for process gas streams only.

The Back Pressure (Regulator) Valve senses the upstream flowing PV Pressure to perform its job. Hence the ISA Symbol for the Back Pressure (Regulator) Valve has an appendage that links the pressure sensed on the top of the Valve to the upstream side of the Valve body or the directly upstream process line.

NOTE: The Setpoint Pressure for the Back Pressure (Regulator) Valve is the upstream process line pressure entering the Valve!

Hey! That doesn't sound right to the ear ... but it is true:

The Upstream Pressure is sensed to determine the set point for a Back Pressure (Regulator) Valve.

The Back Pressure (Regulator) Valve performs two jobs:

The Back Pressure Regulator Valve will be situated downstream of a Vessel like a Separator or Drum. The Back Pressure Regulator will help the Vessel maintain its optimal operating Pressure.

Maintains the desired operating PV Pressure on a Vessel ... like a Separator or a Water Knockout Drum ... which will be situated upstream of the Valve.
Relieves any excess PV Pressure that has built up within the Separator or Knock Out Drum by sending the higher-Pressure fluid downstream of the Back Pressure (Regulator) Valve. Note that this job related to relieving excess "back pressure" gives the Back Pressure (Regulator) Valve its name.

Thanks again to Kimray Inc. for use of this Back Pressure Valve Picture and Cutaway Graphic. The Force of the Upstream Pressure is balanced with the Pressure sensed on the top side of the Diaphragm. Excess Pressure lifts the Diaphragm and allows more flow until the upstream flowing PV Pressure returns to the desired Setpoint. Thus, no 'back pressure' is allowed to build up.

This excellent graphic illustrates the operational differences between Pressure Regulator Valves and Back Pressure Regulator Valves. The gas flowing out of the Cylinder is fluctuating and at a higher flowing Pressure than desired by end users ... like the (unspecified) Process. The labelled Pressure Regulator Valve decreases both the fluctuations in the gas flowrate and decreases the flowing PV Pressure to the downstream flowing Setpoint Pressure desired by the (unspecified) Process. The labelled Back Pressure Regulator Valve maintains the upstream flowing Setpoint Pressure which is also the operating Pressure of the (unspecified) Process Unit. Any excess PV Pressure that builds up in the Process is relieved downstream of the Back Pressure Regulator.

Without a Back Pressure (Regulator) Valve positioned to maintain the optimal operating Pressure, lower Pressure Separators and Knock Out Drums would not be able to optimally perform their duties.

By maintaining the optimum low-range PV Pressure on a pressurized Vessel, and then dumping excess PV Pressure whenever the upstream Pressure Set Point is exceeded, a Pressure Differential ... aka ΔP ... is established which thus makes it possible for the gas to flow out of the Vessel. Every PTOA Reader and Student knows by now ... No ΔP = No Flow!

Back Pressure (Regulator) Valves will be found on vent and flare lines, too. Without a Back Pressure Regulator Valve, it would be difficult to insure forward flow in such low-pressure process service applications.

Process Operators should be able to recognize and distinguish the Back Pressure (Regulator) Valve from a Pressure Reducing/ Regulating Valve on a P&ID. Process Operators should also be knowledgeable of the upstream Set Point Pressure for the Back Pressure (Regulator) Valve and why that Set Point was selected.

For a more detailed understanding of the form and function of Back Pressure (Regulator) Valves access the Kimray You Tube Here.

TAKE HOME MESSAGES: Pressure Reducing/ Regulating Valves and Back Pressure (Regulator) Valves are Special Task Valves which are piped in-line with the main process stream and simultaneously influence the PV Flowrate and its flowing PV Pressure.

Understanding the terms "Upstream" and "Downstream" are prerequisites to fully understanding the respective special tasks performed by the Pressure Reducing/ Regulating Valve and the Back Pressure (Regulator) Valve. Given a specified reference point in the process stream's flow path, "Upstream" refers to the inlet side of the reference point and "downstream" refers to the outlet/discharge side from the reference point.

Pressure Reducing/Regulating Valves:

The Setpoint Pressure for the Pressure Reducing/ Regulating Valve is sensed directly downstream of the Valve. The ISA Symbol for a Pressure Reducing/Regulating Valve has an appendage connecting to the valve's body or process line on the downstream side of the Valve.

The upstream PV Flowrate of the process fluid that is about to enter a Pressure Reducing/ Regulating Valve is fluctuating and at too high of a flowing PV Pressure than the end user needs. The job of the Pressure Reducing/ Regulating Valve is to convert the fluctuating process stream with the higher-than-desired flowing PV Pressure into a more consistent PV Flowrate with a lower flowing PV Pressure.

Process Operators must be able to distinguish a Pressure Reducing/Regulating Valve from a Pressure Relief Valve as both are commonly referred to as a "PRV." Process Operators must be aware of the downstream Setpoint Pressure and why that PV Pressure has been selected as the Setpoint.

Pressure Reducing/ Regulating Valves are needed in distribution systems wherein the generating source of the fluid flows at too high of a Pressure than can be safely used. Examples include the natural gas feedstock to a Gas Turbine (aka the GT's fuel gas) and the instrument air that makes it possible for pneumatic Automatic Control Valves to function.

Back Pressure (Regulator) Valves:

Back Pressure (Regulator) Valves are for gas service only.

The Setpoint Pressure for the Back Pressure (Regulator) Valve is sensed directly upstream of the Valve. The ISA Symbol for a Back Pressure (Regulator) Valve has an appendage connecting to the valve's body or process line on the upstream side of the Valve.

The job of the Back Pressure (Regulator) Valve is to maintain the desired operating PV Pressure ... and thus maintains the ΔP required for flow ... on low-pressure Vessels (e.g. a Separator or a Knock Out Drum) which will be situated upstream of the valve.

The Back Pressure (Regulator) Valve gets its name from its second job; the Back Pressure (Regulator) Valve opens up more when the upstream Set Point Pressure is exceeded. The process stream with higher-than-desired-flowing pressure is thus sent downstream of the Back Pressure (Regulator) Valve, hence relieving the buildup of Pressure.

Back Pressure Regulator Valves will be piped in directly downstream of a Separator in the Overhead Gas/Process Off Gas process line. The effluent process stream could be piped to a fuel gas drum and relieved to the Flare line when needed.

Process Operators must be aware of the upstream Setpoint Pressure and why that PV Pressure has been selected as the Setpoint.

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

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