Design tips for safe use of tank manhole

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Here are some things to keep in mind when designing tank manhole.

Opening and closing manholes falls under 3K work.

Even if it only happens once a year, safety measures such as weight reduction and labor saving are essential in this day and age.

If you leave the problem as a testing ground for your workers, at some point they may cause injury.

Novice engineers tend to design without consideration, but this idea seems to apply to design in general as a way to create worker- friendly designs .

Manholes are heavy and dangerous!

Role of tank manhole

Let’s first check the role of the manhole.

people enter

A manhole is a place for people to enter the tank.

It’s natural because it’s a man and a hole.

Larger manholes are preferable for people to enter, but problems arise when they are too large in terms of pressure resistance and weight.

People enter for equipment cleaning.

Opportunities are rare, but it’s a very dangerous job, so think carefully.

instead of peephole

Manholes can be used as an alternative to peepholes .

Large manhole covers cannot be used to secure items such as pipes and instruments.

If you put glass in such a place, you can give it a function as a peephole.

While it is a place for people to enter and exit during inspections , it can also be used to check the inside during operation .

That’s why we sometimes use manholes as a starting point when deciding on tank layouts and nozzle orientations .

It’s surprisingly important.

Connection of large-diameter pipes

Manholes are sometimes used as connection ports for large-diameter pipes .

Depending on the size of the tank, it is surprisingly difficult to attach a nozzle of 150A or 200A.

In batch chemical plants, the more nozzles, the better, which helps prevent product changeovers and the risk of color mixture.

However, it is not possible to increase the size per nozzle by increasing the number.

Batch-type chemical plants have nozzles that require large diameters for feeding powder and vacuum distillation, but there are cases where such nozzles cannot be installed.

This is where the manhole comes into play.

It’s a negative nuance to use it without stopping, but there are situations where it can’t be helped.

In chemical plants, it is normal to visually check through the observation window when feeding the liquid, and connecting the manhole to another pipe is denying the visual check .

I would like to design it carefully.

Length and width of manhole (tank manhole)

Let’s explain the vertical and horizontal dimensions of manholes.

The following examples are common for vertical and horizontal sizes.

  • 300×400 semi-ellipse
  • 400A
  • 450A

It’s a rough sensitivity, but I think as follows.

300×400don’t want to hire
400ACompromise and mostly this
450AI would like to adopt if possible

I think I’m out of ideas…

Average Japanese size

There is data called “Human body dimension data collection for design” (Ministry of International Trade and Industry, Biotechnology and Industrial Technology Research Institute).

According to this, the following data are interesting.

  • 95% of women have buttocks within 39.9cm
  • The average shoulder width for men is 45.6 cm .

These are data that can be used for things like designing chairs.

Well, the important thing when deciding the manhole size is the shoulder width .

The width of the shoulders is the largest width of a human being.

In the sense of simple design, the basic principle is shoulder width < manhole size .

It seems that the average male height is 45.6cm, so it means that it is not 45cm or 450A .

In reality, there are hidden preconditions such as “I can go in and out comfortably” and “I can’t go in and out while changing my posture” , so I’d like to be able to see through them properly.


300×400 semi-ovals can be seen in old tank manholes .

Even now, it may be used when emphasizing the airtightness of the tank, but it is an exception among exceptions.

It’s a remnant of when Japanese people were small.

When I was younger, I managed to get in and out of the 300×400 size, but now I can’t (laughs).


400A circular manholes can now be seen as standard.

It is large enough for most people to get in and out of , and it has the advantage of being easy to procure and manufacture parts .

I can go in and out somehow…


I also see 450A manholes that are one size larger than 400A.

I want you to adopt 450A by all means! (lol)

When choosing 450A, there is a check to see if there are any problems with the nozzle orientation and tank strength.

Thickness of tank manhole

The concept of thickness is important in manhole design.

Because it is directly related to the weight of the manhole .

This is an important design element in terms of safety during opening and closing work.

Flange weights are summarized in the table below.


JIS10k is excessive

I would like to avoid JIS10k flanges when determining the manhole thickness .

Adopting a blind flange that is determined by the flange standard is easy in terms of procurement, but it is extremely difficult to unconditionally select JIS10k.

In the case of batch chemical plants, the design pressure is often as follows.

  • Tank is FV (Full Vacuum) to atmospheric pressure
  • Piping is JIS10k

Since the piping side is JIS 10k, the nozzle is also JIS 10k, and the manhole is also JIS 10k.

I tend to design like this.

A JIS10k flange weighs 52.1kg at 400A, and a whopping 68.4kg at 450A.

Tank manholes are not installed in low positions like manholes found on roads and in the city, and in many cases they are in rather unstable positions in the sense that people lift heavy objects.

Tank manholes are often placed at waist level so that workers can easily check them visually .

At least JIS5k

Looking at the table above, I think JIS5k is the lightest and most realistic option.

This is undeniable.

But can a person who has actually opened and closed a JIS5k 400A be able to select a JIS5k without hurting his or her heart?

Every time I see him opening and closing the door with a heavy look, I think to myself, ” Isn’t it possible to make it easier? “

I can’t help it because I can’t make it any lighter!

There are really a lot of designers who let go, but I don’t want to be like this.

What is JIS2k?

There is still an option in the world that does not stop at JIS5k flanges, but JIS2k flanges.

However, off-the-shelf products are limited to 450A or more.

JIS5k 400A flange is 41.7kg and JIS2k 450A flange is 48.8kg.

There is a tendency to conclude that JIS2k flanges are heavier and meaningless.

Personally, I would like to make a manhole with a thickness equivalent to JIS2k at 400A .

The lighter the weight, the higher the safety.

light switchgear

Manholes are generally equipped with a light opening/closing device.

There are davit type and hinge type.

The website of Daido Machinery Co., Ltd. will be helpful.

  • tank top plate hinge
  • Tank side plate Davit

It would be nice to use it properly.

Because the chemical plant is too narrow for davits on the tank top.

Davits are safer , so I’d like to use davits as much as possible, but…

I’m worried about the hinges because it’s easy to squeeze my fingers and it’s dangerous for one person.

Material of tank manhole

Let’s also explain the material of the tank manhole.

An image of a stainless steel tank.

It’s mainly about cost.

Is solid stainless steel expensive?

Making a heavy object called a manhole out of solid stainless steel will feel expensive.

Users who want to keep the price as low as possible tend to avoid solid stainless steel.

Stainless steel lining on SS400

SS400 may be lined with SUS304 for the purpose of cost reduction.

You can understand the concept because you can lining the SS400 with a SUS304 plate of about 4mm.

But this lining is quite troublesome.

  • perimeter weld
  • Dot welding in the center
  • news hole

perimeter weld

When lining SS400 with SUS304, welding is the first method that comes to mind.

At least the peripheral part of the end face must be welded.

No problem here.

Dot welding in the center

When welded in the circumferential direction, a strength of about 400A is very weak.

The lining part is easily deformed because nothing is attached to the central part.

It is difficult to say that there are no problems due to the influence of process reactions.

During long-term use, the lining may break due to repeated vibrations.

news hole

It is common to add a hole to let you know when lining is done.

This is for the purpose of providing an air loophole during welding and the purpose of early detection of corrosion .

Air holes are a problem that can also occur in patch plate welding.

Early detection of corrosion is concerned with cracks in the lining.

It is possible that when the lining cracked, the contents remained between the base material SS400 and the lining SUS304, and corrosion progressed without being noticed.

In order to detect this at an early stage, a hole is made in the base material SS400. This is the news hole.

There are many patterns that are stopped with a plug.

amount comparison

How can we compare the cost of lining SS400 with SUS304 and the cost of SUS304 alone?

I have no experience working for a tank production company, so I have only listed an approximate amount, and it is not highly reliable.

Material cost40,000100,000
processing cost30,0000

For the material cost, I chose a general amount that I found on the Internet.

The processing cost is based on the assumption that SUS304 boarding can be manufactured in one day.

Just looking at this, the price difference is only 30,000 yen.

The amount of the tank itself will exceed 2 million yen, so is it worth sticking to 30,000 yen?

It is my opinion that we need to have an attitude of constantly asking questions.

The reason why I have been using SS400+SUS304 for a long time is the price. So keep doing this!

That is dangerous, and is SS400 + SUS304 really advantageous, or is SUS304 useless? I think we need flexibility to reconsider that.

This is a story that you can’t understand unless you listen to the raw voice from the manufacturer.

SUS304 alone is easier to manage materials and processes, so I think it is effective in terms of reducing resources.


Flange information is essential for determining weight in manhole design.

Let’s take advantage of the following pocketbooks that are indispensable for designers.

created by Rinker
¥3,300 (2024/04/15 04:02:28時点 Amazon調べ-詳細)


I explained what to think about when designing manholes for chemical plant tanks.

It summarizes the function, size, thickness, and material of manholes.

Since it has a lot to do with safety, I would like to avoid a pedantic design.

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.