DB(Dia inch) is Poweful tool for plant construction estimate

DB dia inch
記事内に広告が含まれています。This article contains advertisements.

DB (Dia inch) is something you’ve heard before if you’re a mechanical engineer at a chemical plant.

The definition is simple, but not many people are fully aware of the concept of DB and its application.

In this article, I will introduce why DB is important and what you can learn from it.

If you understand the way of thinking around here, you will be able to think from a high perspective and a wide perspective as an owner engineer as well as a project.

Definition of DB (Dia inch)

Dia inch is the length of the weld line.

A weld of 1 inch = 25.4 mm is defined as 1DB (diamond inch).

Why is the expression DB?

D indicates Diameter and B indicates inch.

As shown in the figure below, I am imagining a scene in which the pipe is welded all the way around.

This image looks at the pipe from the side.

Pipe weld line (longitudinal) (Dia inch)

If you look at this in cross section, it will look like the figure below.

Pipe weld line (cross section) (Dia inch)

The idea is to make this “round-the-round welding” a single unit.

The unit of inch is called B designation in Japan . B designation is common in continuous plants.

A designation is an expression such as 25A, 40A, 50A. Batch plants often use the A designation.

Diamond inch is a combination of D and B.

Inch diamond would be BD.

In this blog, we will use diamond inches.

Examples of DB (diamond inch)

Let’s look at some DB examples.

Straight pipe

Welding of 1inch piping is 1DB.

The image below.

1DB (dia inch)

2 DB for 2 inch piping.

When the diameter doubles, the weld line also doubles.

It’s easier to understand with a diagram than with numbers.

Twice as long, isn’t it?

2DB (dia inch)


Welding when connecting straight pipes is considered in one place.

As you can see in this diagram.

1DB (dia inch)

The idea is to add another straight pipe to the first straight pipe.

What about installing fittings such as elbows?

There is always something connected to both ends of an elbow.

Since it is both ends , connect two places.

Elbow (Dia inch)

Connect one end face of the elbow to a straight pipe at one point, and connect the other end face to a straight pipe or the like.

Only this one end face is counted as an elbow, and the other end face is counted separately.


The cheese is the same idea as the elbow.

1inch is 3DB.

Cheese (dia inch)


What are your thoughts on flange welding?

Different companies may think differently.

At least, in the organization I belong to in the company I belong to (troublesome expression…)

Think of it like the diagram below.

Flange (Dia inch)

One-sided fillet welding is applied to two locations on the inner and outer surfaces of the flange.

The idea is to count this minute.

I learned that there is another company that considers flanges with 1.5DB.

The number of welds is actually less because the inner side of the flange does not have a groove.

This is not only the length of the weld line, but also the concept of area (or volume).


I also have trouble thinking about the reducer.

This is because the diameter changes before and after the reducer, and the weld line length changes.

Take a look at the example below.

Reducer (Dia inch)

If you count seriously, it’s 3DB.

Any serious company will pick up the reducer caliber properly.

The company I belong to… In short, I think based on the larger diameter.

Considering a 2inch/1inch reducer as the same as a 2inch elbow, it is a method of counting as 2×2=4DB.

I think that the counting method differs depending on the company.

DB changes with plate thickness

It does not occur in batch plants, but it is a possible problem in continuous plants.

Even with a straight pipe, if the diameter changes, the plate thickness will also change.

As the plate thickness changes, the number of welds also changes.

Let’s take the case of 1 inch and 1 DB as an example, where welding is completed only once.

Thickness 1 (Dia inch)

What if this is about 10 inches and the welding is done three times instead of one time?

Thickness 2 (Dia inch)

If the number of welds is one for 10 inch, it is 10 DB.

This problem occurs as the plate thickness increases.

The company I belong to… for short, this problem has not occurred, so I don’t know what kind of response each company is doing.


If you think of this DB, you have to be careful about the problem of double counting.

Consider the example below.

Combination (Dia inch)

If you seriously count the number of welding parts, it will be 7DB.

Counting the DB from the number of parts gives 8DB.

2 x 2 + 2 x 2 = 8 because there are 2 elbows and 2 flanges.

This is because there is no straight pipe between the elbows.

I think that the way of thinking about how seriously this area is considered varies from company to company.

The company I belong to… for short, double count.

  • Accumulated time to precisely count welded points is not available
  • There are very few combinations such as elbow-elbow (usually a straight pipe is inserted)

This is the way of thinking.

Capital investment and DB (diamond inch)

The method of estimating capital investment changes depending on the scale, accuracy, and timing.

There are many decisions to be made before making an investment decision.

In the initial stage of consideration for investment, the accuracy of the estimate is low, but speed is required.

In this case, the Lang coefficient is used.

It is a way to see various construction costs roughly with “coefficients”.

Configuration of Lang Coefficient (Dia inch)

When the examination stage progresses and you start examining in detail, you have to improve the accuracy of the estimate.

The coefficient method has limitations. That’s where the DB comes in.

Structure of DB (Dia inch)

Breakdown of plumbing work

What kind of process does plumbing work consist of?

  • material processing
  • member welding
  • transportation of pipes
  • Field installation

I think this area is generally defined as “plumbing work”.

Depending on the company, the following items may also be included in “plumbing work”.

  • Create a spool diagram
  • tightness test

There is also a case to go out for on-site installation.

Quotation items to include in plumbing work are not uniformly defined.

This area is the darkness of the chemical plant construction estimate industry.

There is no unified rule…

Positioning of plumbing and welding

As mentioned above, plumbing work is a combination of various tasks.

Then, what should be used as an index when estimating “plumbing work” ?

In the case of piping work, welding work is used as a representative value.

Piping work = Welding work x Z

If you say, “I have to do complicated calculations because there are various things,” it won’t work.

Piping work = material processing x A + member welding x B + piping transportation x C + on-site installation x D

I don’t do these calculations. It is the following image when written in the figure.

Structure of piping work cost (Dia inch)

It is the basis of estimation that it should be represented by an easy-to-understand core indicator.

In the above example, the idea is to combine the coefficients A, B, C, and D into the coefficient Z.

DB is a representative value for welding work

Welding work is expressed using DB.

Since DB is the weld line length and the welding operation depends on the weld length,

If DB is known , the amount of welding work can be known.

The premise around here is a trap if you don’t interpret it correctly.

The reason why welding work is representative of piping work is simply to seek simplicity.

For example, the degree of difficulty of welding work changes depending on the material, and it also changes depending on the shape of the pipe.

A more critical factor would be the caliber.

If it is DB, even if the length is the same, 25A and 50A will be calculated with a 1:2 relationship.

Is the actual work really a 1:2 relationship? People who actually do the work don’t feel that way.

It’s important to think of the DB as a tool that you use to simplify your calculations.

Construction of a batch chemical plant that can be understood from DB (Diamond Inch)

Why is DB positioned so important in chemical plants?

This is because various information can be inferred from the DB, and it can be treated as a major index.

Let’s take a closer look.

Construction amount

The purpose of DB is to estimate plumbing costs.

Piping cost = DB x unit price

This relational expression is the basis of chemical plant construction.

This is because the majority of chemical plant construction work is piping work.

When constructing a plant structure from scratch, civil engineering and construction costs are quite high, but even in this case, the amount of piping work is very large.

Among the Lang coefficients, there are many coefficients for piping work.

For example, at a chemical plant, there is construction with a Lang factor of 4, and at least 1 or 2 is piping work.

This means that 25% (1/4) to 50% (2/4) of the total construction cost is plumbing.

Even if there is no expansion or renovation of the plant, there are many opportunities to expand facilities and extend pipes.

As the construction cost, which is the second highest construction cost after plumbing, is small, the percentage of the construction work that is accounted for by the plumbing is high.

Piping work is the main part of a chemical plant.

Knowing this amount will lead to an increase in the accuracy of the estimate and estimate of the construction cost.

The DB directly influences management decisions on whether or not to make that capital investment.

If you think about it, DB settings are pretty important, but the lack of unified rules is probably the scariest thing about chemical plants.

The world is far from digital and quantitative.

Construction unit price

If you know the DB, you will know the construction unit price of the construction company.

Piping cost = DB × unit price

It is the “unit price” part of this equation.

When estimating construction work to a construction company, it is normal for the DB value to be presented along with the construction cost.

The company I belong to, if abbreviated, can design the piping to some extent in-house, so you can grasp the DB with high accuracy in-house.

In this case, the construction company will make an estimate according to the DB value presented by the ordering party.

Conversely, if the ordering party cannot present the DB, the construction company will estimate the DB.

In either case, the DB has a big impact on the estimate.

The construction amount and DB will be presented by the estimate collection destination, so the estimate collection destination will consider “Construction Unit Price” can be grasped.

Some people are unaware of this impact.

If you know the unit price from the construction company, you will know the “labor unit price” provided by the construction company.

By comparing the labor unit price provided by the construction company with the average labor unit price in Japan, it is possible to estimate the mobilization power of how many excellent workers can be secured.

They don’t hire because the labor cost is too high.

There are companies with this way of thinking, but this will only make them poorer.

While the supervision of the construction company struggles to manage the poor quality of the workers, the burden is also transmitted to the orderer.

It just results in something like this.

I am also the one who has had a hard time.

If the labor unit price presented by the construction company rises, it can be inferred that there must have been some change in the mobilization power and the labor unit price of the industry.

In any case, the labor unit price can be used as data for medium- to long-term management decisions in the work of ordering construction , which tends to be haphazard.

DB has a direct impact on that labor rate.

Construction period

DB refers to the length of the weld line.

This represents the volume of plumbing work.

There is an idea of ​​man-hours , but DB is directly connected to man-hours.

It is possible to infer man-hours from the number of meters of piping, but it is also possible to infer man-hours from the DB.

If anything, the DB can calculate the man-hours with higher accuracy than the number of meters of piping.

How many man-hours can be implemented for 1DB?

This indicates the speed at which the plumbing work will be carried out.

Considering man-hours on a daily basis, is it often understood that 1 man-hour = 〇 DB ?

This is an indicator of how many DBs can be welded in a day.

This is the skill of the welder itself.

It can be said that it is the ability itself that the plumbing company has.

Welding work is the bottleneck in plumbing work , so if you know the speed of welding work, you can know the speed of plumbing work.

If you know the amount of piping work, you can estimate the number of days it will take to do the work.

If you request the same construction company every year, the number of man-hours per DB is roughly fixed, so it is possible to infer the construction period from the DB alone.

That is important in practice.

Aspects of piping work that are not considered in DB (Diamond Inch)

DB is an idea that focuses on welding, but plumbing work does not consist only of welding, right?

Let’s take a look at the elements around it.


Pipe length is not evaluated in DB.

Only weld length is rated.

The two pipes below are both 1DB.

Both are 1DB (Dia inch)

Both pipes are 1 inch.

The top has 1 weld on the 5m pipe and the bottom has 1 weld on the 2m pipe.

Both are 1DB considering only the number of welds.

The only difference is the length of the pipe.

This changes the following elements:

  • Since the number of pipes is different, the material cost will change
  • Since the weight is also different, the workability such as transportation and installation should also be different

However, both are the same 1DB on the DB. the cost is the same.

From the point of view of the people who actually work, this is the part where a sense of unfairness comes out.

There is no other powerful piping estimation method, so it can’t be helped, but what is not considered is not considered.

Let’s recognize that.


The shape of the pipe is also not taken into consideration, as is the length of the pipe.

Different shape (Dia inch)

The caliber is 1 inch.

Both of these two pipes have two weld lines. Both are 2DB.

Which pipe would be easier to install?

The lower straight pipe is usually easier.

It makes a difference when welding in the workshop.

When welding all around a pipe, workability will suddenly deteriorate if there is only one elbow.

Even when installing it on site, the elbow causes a lot of trouble similar to a “wisdom ring”.

It’s easy to use a toy ring of wisdom, but it’s hard to imagine how to install heavy pipes by swinging them around on site.

As an exception , straight pipes that are too long can be difficult to transport.

It’s a problem of not getting on the truck.


Materials are not taken into account on DB.

Consideration for materials will be put on the “man-hours” by a factor.

  • 1DB of SGP piping
  • 1DB of SUS piping

These two “1DB” cannot be the same effort.

SGP 1DB is 〇 man-hours, SUS 1DB is △ man-hours

It is normal to set the man-hours for the DB like so.

Work environment that affects DB (Diamond Inch)

The idea of ​​“unit price” is important when estimating and contracting for construction.

It is unrealistic to build up estimates for a huge amount of construction work one by one.

Using something as a unit, determine the number of man-hours per unit and multiply .

Efforts = Effort Unit Price x Quantity

This is a basic idea in the man-hours of any construction. It’s very important.

When applied to plumbing work

Piping man-hour = Piping man-hour unit price × DB

This is where the DB (diamond inch) comes in. It’s important.

The cost of plumbing work is largely determined by two factors.

Piping work unit price ∝ Worker’s ability, work environment

Using a little more mathematical expression:

Unit price for piping work = Unit price for piping work (worker’s ability, working environment)

I just want to say that it depends on the skills of the workers and the work environment.

The ability of workers depends on the construction company’s mobilization capacity and network in many ways, so it is difficult to discuss.

The labor cost for piping is affected by the work environment.

As a work environment, we divide it into two parts, the workshop and the field.

workshop welding

For welding in the workshop, it would be nice if you could imagine welding in a factory that you often see on TV.

If you search for the words “workshop” or “welding”, you will find a lot of them.

Welding work in the workshop is classified as easy work.

Welding work is roughly divided into the following four types according to the working posture.

  • Downward
  • Sideways
  • upward
  • all around

As the word says, the top is downward.

Downward is the easiest, sideways is fair, upward is difficult.

Classify like this.

The entire circumference is a composite of bottom, side and top.

Almost everything can be welded “downward” in the workshop.

Welding in the workshop is classified as easy work because the simplest class “downward” can cover almost everything.

On-site welding

On-site welding is basically difficult work, whereas workshop welding is easy work.

narrow place

A chemical plant is an element that is particularly likely to fall under this category.

There are many pipes running around, and welding on site sometimes requires an acrobatic posture.

Postures other than downward welding are basically unreasonable.

Welding in this position increases the possibility of weld defects.

Chemical plants are generally small, so they probably include it in the contract conditions for each plant.

That company has a small working space, so let’s overestimate the narrowness of the working environment.

This kind of thinking is reflected in the man-hour unit price.

high place

High places are just as bad as narrow places to work.

  • Scaffolding is required, and it is difficult to lift and transport materials and equipment.
  • affected by rain and wind, especially wind
  • As in a narrow space, you may take an unreasonable working posture.

Field welding is not common in chemical plants.

This is because there is a possibility that dangerous materials pass through the area where the on-site welding is performed.

If even a small amount of hazardous material leaks from the gasket and welding sparks come into contact there, fire comes out.

If there is a large amount of dangerous material in the pipe, the fire will quickly spread over a wide area.


The concept of DB (diamond inch) is very important for plant piping designers.

It is also a good idea to use books such as the following to gain a systematic understanding.

created by Rinker
¥3,850 (2024/03/01 02:40:22時点 Amazon調べ-詳細)

Related information

Related information


I explained the concept of DB (diameter) used in a batch chemical plant.

We introduced the definition of DB, its relationship with capital investment, and the concept of DB as a representative value for construction work.

Don’t be overconfident, let’s hold down the points to be careful and consideration properly.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (Comments are at the bottom of this article.)

*We will read all the comments and reply seriously.