Introducing an example of piping design around a pump.
Pumps are used everywhere in chemical plants, but the piping design around the pumps is surprisingly problematic.
The reason is spatial space.
When you simply build from vacant land, you have a lot of space, and it is possible to create a design that is faithful to the basics.
When expanding or renovating a building after construction is complete, there is almost always an increase in the number of pumps, and at that moment the degree of difficulty in piping design rises sharply.
We have summarized the design around the pump so that it can be used in a wide range of applications, from basics to applications.
Piping design is something that mechanical engineers should be particular about.
- Piping parts around the pump
- discharge line
- suction line
- circulation line
- non-return valve
- Related article
Piping parts around the pump
The piping parts that should be attached around the pump are shown below.
We will explain the purpose of each element.
A pressure gauge is a typical instrument that indicates the operating status of a pump.
A local pressure gauge is sufficient.
It does not use a pressure transmitter that feeds into the DCS.
For processes, it is common to use a diaphragm pressure gauge.
With the pump installed and running, color the range of indicated values on the pressure gauge of the pump,
By monitoring whether it is operating within that range, it is possible to monitor the pump on a daily basis.
It is not impossible to use it for capability verification together with the performance curve, but it will be difficult.
The pressure gauge should be taken out from the pump outlet like a branch .
The vent line ( air release ) for the pump can also be constructed from the branch.
It is also possible to have a pressure gauge on the header, but it is better to save the header for piping space.
A thermometer is used to measure the temperature of the fluid being pumped.
This is a rather special case.
It is more common to not have a thermometer attached.
If the power of the pump is directly applied to the fluid as heat when the pump is operated in the closed state, a fire or explosion may occur depending on the type of fluid.
Scary characteristics unique to chemical plants .
Thermometers may be installed to detect temperature rises early.
In this case, create a system that interlocks the pump based on the set value of the thermometer.
It is best to place the thermometer in a position where it can properly measure the liquid temperature.
A T-pipe connecting the pump outlet header is ideal.
If difficult, take a branch from the outlet in the same way as the pressure gauge. I don’t really want to do that though.
The suction port of the pump is almost always provided with a liquid drain .
I have an image of the tank pushing into the pump, but the suction type is the same.
Install a shut-off valve on the primary side of the drain valve.
One point is that this is for pump maintenance.
Another point is to use for removing liquid during line cleaning.
In order to minimize the amount of liquid drained, it is good manners to install a valve close to the pump suction port (laughs)
In some cases, a liquid drain is attached to the pump outlet.
This is used when you do not want the pump to rotate in reverse or when you want to drain the liquid at the pump outlet quickly .
The liquid drain at the pump outlet can also be used as an air intake port .
It is a good idea to attach a release valve to the branch of the pressure gauge.
Make sure to attach a single pipe to both the outlet and inlet of the pump .
This is the biggest reason for pump maintenance .
To minimize the number of pipe flange disconnects.
The suction port has this function with a shutoff valve, but the outlet may have no flange up to the header valve.
In this case, the pump casing may not be able to be disassembled unless all flanges of the outlet header valve are separated.
In that case, it would be better to put a single pipe between the pump outlet and the header.
If a flexible joint for vibration absorption is provided, it can be used instead of a single pipe.
If you plan to use the line immediately after the piping has been remodeled, install a temporary strainer to remove dirt from the piping work.
To attach a temporary strainer, it is convenient to attach it to a single pipe.
Disassembling the flange of the pump body may seem simple, but it is tricky.
If you tighten the flange incorrectly, the pump may become distorted and become unusable.
There is little resistance to disconnecting and reconnecting piping parts that are less affected.
This single tube is supposed to be basic, but surprisingly there are many things that are not done well .
This section explains the concept of piping design for pump discharge lines .
This is the world of pure pressure loss calculation.
Determine the piping diameter based on the pump flow rate and pressure loss .
If you use the standard flow rate, you can skip a lot of the calculations.
There are some factors that can be considered to minimize pressure loss, such as the orientation and arrangement of headers , but they are not given much importance except in special cases such as slurry.
This is because the pressure loss does not change much even if the piping layout is slightly changed.
This section explains the concept of piping design for the pump suction line .
The diameter of the suction line is one size larger than the discharge line.
This is to omit pressure loss calculations.
In old plants, we often see cases where the suction line and discharge line are the same size .
This is not a problem if you are continuing to use the same product without any expansion or renovation, but if you introduce a new product, you will have to think about it a bit.
In recent years, there has been an increase in the number of liquids with troublesome properties such as high slurry properties, high viscosity, and easy formation of crystals.
They probably have no choice but to make last-minute process designs.
You can’t create a new reaction path so easily.
In this case, the problem of cavitation and NPSH arises.
In that case, if you increase the suction line by one size, it will not be a problem basically.
First, I will introduce the concept of circulation lines.
The circulation line looks like the one below on P&ID.
Three points to keep in mind here:
- Place the circulation line in front of the pump header
- The diameter of the pump suction line is one size larger than the pump header.
- The circulation line should be one size lower than the pump header.
Even if this idea was unified when it was first constructed, it would become quite unreasonable if it was repeatedly remodeled.
the front of the header
Place the circulation line at the very front of the header .
This is because the circulation line is an absolutely necessary element of the pump header.
The destination to which the pump is sent will change as the building is expanded and renovated .
Once the circulation line is installed, it basically remains unchanged.
Based on this idea, the circulation line is placed at the very front , which is automatically determined once the pump position is determined .
The assumption is that pumps are unique to tanks , but this is increasingly being overturned.
The tank and pump should be fixed like this, but there are cases where it is used like this when another product is introduced after repeated extensions and renovations.
|(do not use)
For some products, Tank A itself is not used, Tank B is used, but Pump B is inappropriate, so Pump A is used.
Depending on the capacity such as flow rate and lifting height and the material, such a case is quite possible.
From pump A ‘s point of view , if you think that the circulation line is fixed to tank A , there will be cases where it circulates to tank B.
The more renovations and additions are repeated, the more the ideology becomes unsustainable.
1 size down
The basic rule is to reduce the diameter of the circulation line by one size .
This is also a pressure loss issue.
Control will be difficult if the pipe size is the same as the destination pipe diameter.
The problem is that when a certain amount of water is sent while being circulated, too much water flows into the circulation line where there is less pressure loss .
Cases often occur in which sufficient control cannot be achieved by simply adjusting the opening of the destination flowmeter’s regulating valve and the circulation line’s valve.
In that case, I think it would be better to go down one size from the beginning .
It can easily change depending on the idea of whether to send it while circulating or to send it in one pass without circulation.
The arrangement of headers also requires some consideration.
A typical example in a batch-type chemical plant is when the same pump is used to separate liquids in tanks by changing the destination.
The oil layer , which is the process liquid, is placed in the middle of the header, and the water layer, which is waste water , is placed at the end of the header.
This takes into consideration the cleanability of equipment cleaning rather than the cleanability after liquid separation .
In liquid separation, the liquid passes through the header in the order of water layer → oil layer .
The washing liquid sent after sending the oil layer is usually the same solvent as the oil layer .
In this case, you wouldn’t be conscious of where the oil feed and water feed are located on the header .
On the other hand, when cleaning equipment, it is common to wash with water at the end.
Washing wastewater is normally documented in the same route as process wastewater.
It is convenient to bring the water to the end of the header so that the end of the header can be washed.
Header orientation and workability
I will explain the orientation and workability of the header.
It’s a part related to the factory layout, and it’s easy to divide thoughts.
Emphasis on on-site workability
It is a basic type that emphasizes on-site work.
This is common in batch chemical plants.
There is a reactor in the part entered from the passage, and a pipe is connected from the bottom of the reactor to the pump.
The case where the suction nozzle of the pump is located on the passage side is called the field work-oriented type.
- Suction piping becomes detoured
- Easy to inspect the pump suction port
- Separation and blowing operations become easier
- Creates dead space
It has these characteristics.
As for the detour of the suction piping, it is faster to look at the case below that emphasizes motor maintenance.
Inspecting the pump suction port is easier because the piping can be removed from the passage side.
It will also make draining work easier.
In a batch chemical plant, there are several manual operations related to pumps, and operations such as liquid separation and nitrogen blowing are performed on site.
Even if you use a camera or conductivity meter to make a judgment, or install an automatic valve and blow in sequence, the work of monitoring on site remains.
This case has the disadvantage of easily creating dead space .
In the diagram above, it is the gray area between the suction pipe and the pump .
There is little room for new equipment or plumbing here. It’s a waste.
Emphasis on motor maintenance
In the motor maintenance focused case, the pump position is reversed from the on-site work focused case.
The motor is out on the aisle side.
You can see that this case has the shortest suction pipe .
Since the motor is located on the aisle side, it is also suitable for periodic inspection of the motor.
On the other hand, there is a disadvantage that you have to go one step ahead of the aisle in order to perform liquid separation or blowing operations.
This is generally the case for continuous plants .
Recently, we have decided that this case has a total merit even in batch plants.
Continuous parts such as outdoor tanks are also actively adopting this type in batch plants.
Let’s consider a case where two pumps are installed in parallel .
This is becoming more common in batch chemical plants.
Two pumps often share a header .
When sharing headers, their placement is highly constrained.
If you only have one pump, you can change the direction of the header to some extent,
- Operate from the aisle side
- Operate by entering through the aisle
You can choose from 2 patterns.
When two pumps share a header, there is no such option, and it is normal to connect the two pumps as quickly as possible.
Basically, the only possible pattern is to work from the aisle .
In this case, there will always be dead space.
Whether it is a type that emphasizes on-site work or a type that emphasizes motor maintenance, dead space has a considerable limit.
Adding automatic valves is a common occurrence in batch chemical plants.
Since the number of destinations has increased, I will increase the number of automatic valves for the time being.
I will make an estimate like this and write down the P&ID.
This is a picture of adding piping and automatic valves. It won’t feel too strange.
However, adding an automatic valve will increase the cost including modifying the valve body and software.
Let’s think about this for a moment.
You may be able to find an escape route like the one below.
There is a case where there is an automatic valve in the header on a line that is not used for a particular product , and you can handle it by removing a branch from it.
This will complicate the shape of the header.
This changes the basic idea of extending the headers in the same direction keeping the distance between adjacent pipes of the header constant .
The maintainability of automatic valves is also becoming worse.
It is often adopted in cases where lowering the initial cost is a priority , even if such sacrifices are made .
I would like to be more aware of this when writing the P&ID.
Check valve problems are often a problem with pumps .
If a check valve is installed in the discharge line close to the pump, liquid will inevitably accumulate due to the structure of the check valve.
The purpose of installing a check valve at the pump outlet is to prevent the pump from rotating in the opposite direction .
When the pump is stopped, the liquid located higher than the pump will fall under its own weight.
Rotating the pump in the opposite direction to the feeding direction may damage the pump.
- The screw-in impeller comes off the shaft.
- Reverse current flows through the motor
Recently, many pumps have taken measures to prevent this from happening, so it shouldn’t be much of a problem.
Installing a check valve may cause problems .
Liquid may accumulate on the upper side of the check valve and gas may accumulate on the lower side, making it impossible to fill the pump with liquid .
Even if you try to put liquid into the pump while gas is accumulated, the liquid will not come into the pump.
All you need to do is compress the gas part a little.
If the pump impeller is not filled with liquid, the pump will not start even if you try to start it.
If you install a check valve, you will need to think about how to vent the gas .
throw away valve
It’s a simple way of thinking.
Place the discard valve.
The amount of liquid pooling can be reduced only to the inside of the check valve and the T-pipe portion of the piping.
Normally this is OK
Even if gas accumulates in the pump chamber, it is OK if you rotate the pump a little.
Since the amount of liquid above the check valve is small, we expect that the gas will flow to the top, displacing the check valve.
A bypass is almost the same as a waste valve.
In the case of a disposable valve, you must open the blind flange and drain the liquid using a bucket, etc.
In the case of bypass, you can solve the problem by simply opening and closing the valve slightly without preparing a bucket.
The purpose of installing a check valve is to prevent the pump from rotating excessively in reverse, but it does not mean that the reverse rotation will be reduced to zero .
If you open the valve even a little to let the gas escape, you should be able to start it up.
Since it is a hassle to open the bypass every time, the idea is to create a subtle loophole using an orifice, etc.
This case is rarely seen in batch systems.
Since the pump does not operate continuously, there will always be times when the pump is stopped during batch operation.
Here, if only an orifice is attached, there is a possibility that liquid will flow back from another system.
The idea is to send the liquid above the check valve using a nitrogen blow .
With nitrogen blowing, there is little point in using a waste valve or bypass.
Nitrogen blowing is highly recommended as it is easy to automate .
Problems with check valves occur not only in individual piping but also in collective piping. It’s troublesome.
Consider the case where multiple pump outlets merge into one collecting pipe.
Let’s assume that there is a check valve next to the pump.
If you simply join two lines together, you will get an image like the one below.
If the pump outlet check valve is in a relatively low position and the collecting pipe is in a high position, liquid will accumulate at the check valve outlet of each line .
It will be difficult to remove liquid and gas, and nitrogen blowing will be necessary.
Raise the pipe a little
Modify the piping line a little to make it look like this:
Liquid from the two lines is less likely to accumulate in the check valve. It will give you the illusion that the problem is solved.
However, the liquid actually accumulates.
This is because the branch pipe of the collection line is not necessarily located at the highest point in the line .
In most cases, the highest point will be somewhere in the piping from the collection line to the tank (in front of the tank).
In this case, this method can only be expected to have the effect of slightly reducing the risk of leakage from the check valve.
Place the check valve in a high position
A practical solution is often to install the check valve in a high position.
As a result, the following effects can be expected.
- The probability that liquid from the collecting pipe will leak into the check valve is low.
- Even if gas accumulates at the rising edge of the pump outlet, there is a high probability that the pump will be filled with liquid.
- Even if the liquid is drawn down by its own weight, the amount of reverse rotation of the pump is small.
Although there remains the problem that the check valve is located in a high position and is difficult to maintain, many operational issues can be overcome.
Piping design is very important knowledge for mechanical engineers of chemical plants.
It is especially important to study about pumps, as it is easy to fail in the area surrounding them.
We explained the piping design around pumps in batch chemical plants.
A combination of pressure gauges, gas vents, liquid vents, single pipes, etc. is required.
It explains how to design piping by dividing it into suction, discharge, and circulation, and also touches on the troublesome problem of check valves.
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.)
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