Let's go steady again like we used to do
Say you're ready and then we can start anew
Seven days we didn't speak
It's been a long and lonely week
Come on, baby, let's go steady again

("Let's Go Steady Again," by Neil Sedaka, 1963)

 

A CONTINUOUS, STEADY PV FLOWRATE RESULTS IN STEADY PV LEVEL

The difference between "manufacturing" and "processing" was explained way back in the PTOA Orientation.

A manufactured product requires parts and pieces to be assembled into the final product.

Processing facilities use energy in its various forms (Pressure, Heat, and Flowing Fluids) to continuously upgrade feedstocks into more valuable final products.

 

Gas, Liquid, and Multi-Phase fluids flow through the process lines of a processing facility as Feedstocks and Intermediate Products on their way to being upgraded into more valuable Final Products.

Note that the concept of "Level" does not exist while the fluids are flowing through the Piping Network.

Only when Liquid fluids are collected in some type of large-diameter Stationary Equipment does the PV Level exist and is measurable.  These Liquid fluids are only temporarily collected because the inlet valve and outlet valve of the continuously collecting container are both open.

The nearby graphic of a Distillation Column illustrates that Liquid Levels have accumulated in 3 Reflux Drums. The Reboiler would also have a Liquid Level that would be controlled.

Towers and Columns are large-diameter Stationary Equipment which are designed to continuously separate components of the feedstock into more desirable or useful products.  A Liquid Level will collect in the bottom of a Tower or Column, too.

Some of the other large-diameter Stationary Equipment specifically purchased and installed to accumulate flowing fluids are called Tanks, Drums, Separators, Receivers, Vessels, and in the appropriately named "Accumulator."

While a Liquid Level accumulates in these large-diameter Stationary Equipment, the kinetic energy of the flowing fluid is converted into the hydrostatic Pressure energy featured in PTOA Segment #145.

Gas fluids that may be entrained in the process stream or intentionally flashed out of the process stream are vented at the top of whatever large-diameter Stationary Equipment the process stream has flowed into.

While the plant is up and running (a.k.a. "online"), continuously flowing fluids are collected in the accumulating hardware described above. Only because the Process fluid flows into a container can the PV Level of the fluid be detected and measured.

The amount of residence time the Liquid fluid has in the accumulating hardware is directly related to the PV Flowrate of the process stream that enters the vessel and exits the vessel.

Each molecule of the Process Stream awaits its residence time before exiting as effluent from the vessel.

Process Stream fluids flow continuously through the pipes until the next planned maintenance interval. A "planned maintenance interval" is known as a "Turnaround." A Turnaround requires stopping the flow of feedstock to the processing unit as part of a plant shutdown.

 

When Flowrate In > Flowrate Out.
(">" means "Greater Than")

An increase in the PV Level indicates that the PV Flowrate of the Process Stream entering the collecting Vessel is greater than the PV Flowrate of the same Process Stream that is simultaneously exiting the Vessel.

More Fluid entering the collecting Vessel is a temporary condition that must be monitored closely.

If the PV Level continues to rise with no simultaneous increase in the Liquid's effluent Flowrate, the Tank will eventually overflow.

 

Flowrate In < Flowrate Out.
("<" means "Less Than")

A decrease in the PV Level indicates that the PV Flowrate of the Process Stream entering the collecting Vessel is less than the PV Flowrate of the same Process Stream that is simultaneously exiting the collective Vessel.

Less Fluid entering the collecting Vessel is a temporary condition that must be monitored closely.

If the PV Level continues to fall without a simultaneous increase in the Inlet Process Stream's Flowrate, the collecting Vessel will eventually become empty.

More likely than not the hardware downstream from the accumulating Stationary Equipment is a pump. Left uncorrected, the PV Level of the accumulating vessel (aka "Tank") will decrease to the height of the downstream pump's Suction Line and the pump will cavitate. Cavitation was featured in PTOA Segment #171.  How to prevent Cavitation from happening was featured in PTOA Segment #172.

 

UNSTEADY STATE (AKA "TRANSIENT STATE") DEFINED

Both the above situations are examples of the process operating in the Unsteady State condition.

Otherwise stated...

Unsteady State infers the PV Level in a Tank or other type of accumulating hardware is either increasing or decreasing.

There are certain process scenarios in which Unsteady State is expected:

• 

  The Unsteady State (aka Transient State) is on the left. The Container is filling up. The Steady State is on the right. The flow into and out of the container are equal.

  Shutting Down a Process Unit in preparation for a Turnaround maintenance interval. The feedstock to the process unit will be gradually reduced to zero. This is the PV Flowrate In LESS THAN PV Flowrate Out situation described above. Eventually all tanks and other types of collecting vessels will be drained or attain a minimum Liquid Level.  This liquid will have to be safely removed prior to entry into the vessel.

• Starting Up a Process Unit after a Turnaround maintenance interval. Special process lines only used during a Startup will be used to fill the bottoms of Columns, Towers, Overhead Separators, Reboilers, etc. to a minimum start up level. The Liquid fluid used for the Startup will be somewhat like the Feedstock or intermediate Product expected to be produced during Startup.

• The occurrence of an unplanned Emergency Shutdown wherein the automatic control scheme will open and close valves for a soft landing.  Typically, the Process Unit is designed to get to a safe, yet still "hot" state while the Process Operators determine the problem and fix it online ASAP.  Otherwise, the Process Unit will be brought to a "cold shutdown" and an unplanned maintenance interval will become everybody's priority.

 

STEADY STATE DEFINED

The goal of any Process Unit Start-Up is to achieve and maintain "Steady State."

Just like the flight of any airplane, the emergency incident is more likely to happen during Take Off (Plant Start-Up) or Landing (Plant Shutdown). The incidence rate of a plane problem once the plane is at cruising altitude is much less (if a proper P/M schedule had been followed).

"Steady State" is achieved once the Liquid Levels in the Process Unit are not changing and therefore are Steady.

Steady State means the volume of Feedstock(s) flowing into the plant matches the Volume equivalent of Final Products flowing out of the plant.

The desired Intermediate Products and Final Products can be produced once the process has achieved Steady State.  The Control Board Operator will notify the Tank Farm that the process unit products can be directed away from the "Slop Tank" and into the desired Final Product tank.

Once Steady State is achieved the plant is producing Final Products made to specification.

Being in the Steady State makes it possible to predict when the continuously produced products from a processing facility will be available for sale.

Being in the Steady State also makes it possible to determine the staffing needed to operate the plant as well as know the utilities required to support the plant.

By all means, let's go Steady!

 

THE TYPICAL TANK LEVEL CONTROL LOOP CONTROLS THE TANK EFFLUENT FLOWRATE

Obviously, the Unsteady State condition that can lead to overfilling a Tank or sucking the Tank dry must be avoided.

The typical Tank Level Control Scheme controls the PV Flowrate of the Liquid exiting the Tank (or other accumulating container).

The nearby schematic illustrates the typical Level Control Loop:

• 

  Note that the Level in the Tank is measured and adjustments are made by increasing or decreasing the Flowrate from the Tank via a Level Control Valve (not a Flow Control Valve). Otherwise stated: The Flowrate from the Tank is used to adjust the Level to the desired Setpoint.

  The Tank Level is sensed and measured by the Level Transmitter (LT).

• The Output Signal of the LT is the input signal to the Level Controller (LC). The measured Level is compared to the Set Point desired.
• Assuming the measured Tank Level is Too High, the output signal from the LC to the Level Control Valve (unlabeled LCV) will open the LCV up more until the outlet PV Flowrate from the Tank increases, which eventually causes the desired PV Level Set Point value to be attained.
• Assuming the measured Tank Level is Too Low, the output signal from the LC to the Level Control Valve (unlabeled LCV) will close the LCV a little more until the outlet PV Flowrate from the Tank decreases, wlhich eventually causes the desired PV Level Set Point to be attained.

 

 

PV FLOWRATE CAN CAUSE PROBLEMS ... BUT NOT PV LEVEL

Guess what?

An accurately measured and displayed PV Level is never the root cause of a process problem.

The PV Level is the symptom of a processing problem, never the cause of a process problem. Process Operators need to look at upstream PV Flowrates, PV Pressures, or PV Temperatures (and perhaps a combo when Gas fluids are involved) to find the root cause of a process problem.

BIG HUGE CAVEAT TO THE ABOVE OBSERVATION: An inaccurately displayed PV Level most certainly CAN cause catastrophic processing problems.  Albeit too many real-life processing catastrophes could be used as examples, the more entertaining version is depicted in the Hollywood movie The China Syndrome.

 

CONTINUOUS PROCESSING VERSES BATCH PROCESSING

This PTOA Segment featured Continuous Processing. The other type of Processing feedstocks into more valuable Final Products is Batch Processing.

Batch Processing is a hybrid between manufacturing and Continuous Processing.

Like manufacturing, Batch Processing follows step-by-step procedures. Like Continuous Processing, Batch Processing uses energy in the form of Temperature, Pressure, and Flowrate to upgrade raw materials into more valuable Final Products.

Paint is made in batches.  So is beer.

Here are some examples regarding how Batch Processing is different from Continuous Processing:

There's just so much red paint needed at any particular time. Better to clean everything out to make a different batch of blue.

And the fermentation process involving yeast is required to make beer.  As of this date, no continuous fermentation process exists because ... although yeast can be encouraged to complete their step in the beer-making process ... they have stubbornly shown Humankind that they shall complete the step on their own timetable.

 

TAKE HOME MESSAGES:  The PV Flowrate/PV Level relationship does not exist until the PV Flowrate is accumulated in a large-diameter piece of Stationary Equipment. Only when temporarily within the Stationary Equipment does the PV Level exist which makes the PV Level possible to measure.

Examples of large-diameter Stationary Equipment include Tanks, Drums, Separators, Receivers, Accumulators, Towers, and Columns. The flowing fluid converts its kinetic energy into hydrostatic energy while it is in the Stationary Equipment. 

The Residence Time of a flowing fluid is the time that it takes a molecule of Liquid to flow through a large-diameter piece of Stationary Equipment.  The inlet PV Flowrate and outlet PV Flowrate of the fluid that is flowing through the accumulating Stationary Equipment determines the Residence Time of the continuously flowing fluid and consequently, the desired PV Level of the contained Liquid. 

The desired operating status is Steady State wherein the PV Flowrate into a large-diameter Stationary Equipment is equal to the flow out of the large-diameter Stationary Equipment. At Steady State there is no change in the PV Level.

Steady State makes it possible to predict when products will be ready for sale and the manpower and utilities required to produce the products.

Unsteady State (a.k.a. Transient State) is anytime a Liquid Level is increasing or decreasing. Both tank overfilling and downstream pump cavitation can result with the PV Flowrate/PV Level Relationship is not steady. 

The Final Control Valve on the typical PV Level control loop opens and closes the effluent PV Flowrate to achieve the desired PV Level Setpoint. The effluent PV Flowrate is the flowrate of the Liquid exiting the accumulating Stationary Equipment.

©2023 PTOA Segment 0243

PTOA PV FLOWRATE FOCUS STUDY AREA
THE RELATIONSHIP OF THE PV FLOWRATE WITH PV TEMPERATURE, PV PRESSURE, AND PV LEVEL

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