Knowing the standard flow rate makes piping design much faster.
Just having this way of thinking is useful in various situations related to chemical plant engineering, such as estimation, design, process analysis, and troubleshooting.
It’s easy as knowledge, and a big difference comes out just by knowing or not.
Without a computer or calculator, I can’t consider it, so I can’t give an immediate answer at the site and take it home.
You can also be freed from such things and solve problems that occur on the spot on the spot.
Not only does it save working hours, but it also greatly increases the trust of the manufacturing department.
Relationship between standard flow rate and flow distribution in piping
Introduces the relationship between the average flow velocity and flow velocity distribution in a pipe.
Mechanical engineers of chemical plants basically speak in terms of average flow velocity when designing.
In fluid mechanics, there is a story about flow velocity distribution.
The flow velocity distribution becomes a topic only when there is a topic related to heat transfer.
The average flow velocity will be used to evaluate the validity of the piping design and process, so let’s have a way of thinking.
Standard flow rate in pipe
The definition of average flow velocity in a pipe is:
$$ U_a =QA=Qπ\frac{D^2}{4} $$
Ua is the average flow velocity in the pipe, Q is the flow rate, and D is the pipe diameter.
It simply calculates “flow rate”/”cross-sectional area”.
Very important.
You also use this average flow velocity in your discussion of pressure loss calculations, don’t you?
It is good to imagine the flow in the pipe as follows.
The average flow velocity is different from the actual flow velocity distribution, but it is a very useful way of thinking to easily perform various calculations and take equipment measures.
simple is best.
Velocity distribution
Actual flow velocity distribution in piping is different from average flow velocity.
In the world of fluid mechanics, most of it is explained mathematically and physically.
That study is also important, but in fluid dynamics, it is a study that talks about the fact that “the actual flow velocity distribution is different from the average flow velocity.”
I think it’s important to have a quick overview.
There are times when it is difficult to understand what you want to say because there are a lot of formulas.
An image of the actual flow velocity distribution is shown.
- Wall of piping → flow velocity is small
- Center of pipe → high flow velocity
This is what I want to say in the image diagram.
The velocity distribution actually changes with the magnitude of the velocity itself.
The distribution is different between laminar flow with low flow velocity and turbulent flow with high flow velocity, and the image above shows the state of laminar flow.
In the case of laminar flow, it is possible to obtain the velocity distribution physically by hand calculation, but in the case of turbulent flow, it is not possible.
Turbulent flow becomes a world simulated by a computer.
The job of a mechanical engineer in a chemical plant is not at the stage of modeling and simulating complex piping shapes.
Complete the impending construction.
We are in a position to proceed with this project.
Let’s ask the specialized team for the simulation.
As a general theory, turbulent flow also has in common that “the flow velocity is small at the wall surface and the flow velocity is large at the center”.
Standard flow rate for liquids
Let’s start with the average flow velocity in a liquid pipe.
clean process fluid
When the process liquid is pumped, it is about 1.0 to 2.0m/s.
The biggest basis for this decision is static electricity charging.
Most of the process liquids are class 4 hazardous materials, and if the flow rate is too fast, static electricity will continue to accumulate. Super dangerous.
In particular , for highly charged substances, the flow velocity may be reduced to about 1.0 m/s or less.
utility
For utilities that are water-based rather than process liquids , the flow rate may be a little higher.
I often “work hard” up to about 2.0 to 3.0m/s.
If the flow velocity is too high, the pressure loss will be too high, which is not economical.
high viscosity liquid
When pumping high-viscosity liquids, reduce the flow velocity to 0.3 to 1.0 m/s.
In batch-type chemical plants, even high-viscosity liquids are limited to around 100cP.
Higher viscosity liquids are basically not handled.
The higher the viscosity, the greater the pressure loss, so the flow velocity is reduced.
gravity flow
For gravity flow of liquids, 0.3m/s.
It is necessary to increase the pipe diameter and reduce the flow rate compared to sending with a pump, so be careful.
- Clean process liquid 1.0 to 2.0m/s
- Utility liquid 2.0 to 3.0m/s
- High viscosity liquid 0.3～1.0m/s
- Gravity flow 0.3m/s
Standard flow rate of gas
For gases, divide into steam and gas.
vapor
For steam, set the flow velocity to about 20 to 30m/s.
I am concerned about pressure loss and limit the flow velocity.
If the pressure loss is too high, the saturation temperature will drop, resulting in steam with poor heat transfer performance.
30 m/s for long-distance piping such as outside the plant where there are few bends.
20 m/s for locations with many bends in the piping, such as around equipment inside the plant.
You will have a lot to choose from around here.
gas
For gases, keep the flow velocity below 10 m/s.
In a batch-type chemical plant, when it comes to gas, most of it is a distillation system.
- atmospheric distillation, to avoid the surroundings of the equipment becoming a pressurized tank or first class pressure vessel
- vacuum distillation, the capacity of the vacuum pump is not excessive
In addition to this, the same flow velocity design is used for the intake of exhaust gas and the exhaust air of the air conditioner.
This is the concept of pressure loss to the extent that it has no effect.
- Steam 20-30m/s
- Gas 10m/s
Standard flow rate of slurry
For slurry liquid, the flow velocity is suppressed to about 1.0m/s.
It’s hard to generalize because it depends on the characteristics of the slurry, but neither too slow nor too fast is bad.
If it is too slow, the slurry will settle in the piping and there is a high risk of clogging the piping.
If the speed is too high, the pressure loss will naturally increase, and it will also cause the impeller to wear off inside the pump.
Of course, the risk of static ignition also increases.
- Slurry 1.0m/s
Standard flow rate for gases containing liquids and solids
I will explain the average flow velocity when liquids and solids are included in gas.
Gas is not just steam or gas, but clean air or dirty? Air, etc.
Examples of such cases include:
- Fill drums with hazardous materials, suck hazardous materials out of drums
- It collects powder when it is filled into drums, and feeds equipment from powder
In a nutshell, it is the case of handling objects in an open system.
Designed to collect hazardous materials to prevent exposure to humans and spread to the environment.
upstream and downstream
Upstream and downstream, the average flow velocity is about 10m/s.
This is so that liquids and solids can be entrained.
Conversely, if it is about 1 m/s to 5 m/s, it will fall downward without being accompanied.
Whether or not to allow liquids or solids to flow upstream or downstream depends on the case.
If a fan is used for suction, the flow velocity should be designed to slow down as the fan should not be mixed with liquids or solids.
Specifically, increase the pipe diameter.
Conversely, if you want to suck in an object handled in an open system, aim for the entrainment effect.
The hose is installed near the drum can, but it can be said that the purpose of not increasing the diameter of the hose is to achieve the effect of “accompaniment”.
horizontal flow
The horizontal flow is also designed to be around 10m/s.
Liquids and solids are entrained in gases.
Horizontal flow is the same idea as upstream and downstream.
If the flow velocity is slow, about 1m/s, it will not be entrained and will accumulate at the bottom of the pipe.
Accumulated liquids and solids narrow the pipe cross-sectional area and increase the actual flow velocity, so liquids and solids are entrained.
If the piping layout allows for the safe removal of liquids and solids accumulated in the piping, the larger the diameter of the piping, the better.
This is because if the pipe diameter is made too small and the pressure loss is too high, the design air volume cannot be secured.
Increasing the pipe diameter increases the cost, so a well-balanced design is important.
- Gases including liquids and solids 10m/s
reference
Piping design mainly involves pressure loss calculations.
The standard flow velocity is the basis for the pressure loss calculation. The following book will be helpful for pressure loss calculation.
Related article
lastly
I explained the average flow velocity in the pipes of a batch-type chemical plant.
They are divided into liquids, gases, slurries, and entrained gases.
There may be some differences depending on the company and factory, but when considering pressure loss and economic design, the values will be about the same.
Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)
*I will read all the comments and answer them seriously.