Types of Piping Plants -02

PG Diploma in Piping Engineering – Topic 3

Pulp & Paper Plant and Fertilizer Plant – Process & Piping Explained

In many industries, large process plants look very complicated from outside – thousands of pipes, tall columns, tanks and rotating equipment. As piping engineers, our job is to understand what is flowing inside those pipes, at what pressure and temperature, and how the process works from start to finish.

In this lesson we focus on two very important process industries:

• Pulp & Paper Plant
• Fertilizer (Urea) Plant

1. Pulp & Paper Plant – Overview

The main product of a pulp & paper plant is paper – which can be writing paper, packaging paper, tissue, newsprint etc. The raw material is usually wood, agricultural waste (bagasse, straw) or recycled paper. The key process inside the plant is to convert this raw material into a pulp slurry and then form it into a continuous paper sheet.

In the picture of the real plant, you can see a long paper machine with multiple rollers, sprays, and walkways. The main fluid handled in piping is:

• Pulp slurries (fiber + water) at different consistencies or densities
• Clean water (process water, cooling water, washing water)
• Chemicals (bleaching agents, additives, dyes, sizing chemicals)
• Steam and condensate for heating and drying sections

Most of these fluids are at low to moderate pressures but wide range of temperatures. Some chemical lines and steam lines may be at higher pressure and need special material and design.

2. Process Flow in a Pulp & Paper Plant (Explaining the Diagram)

The schematic diagram you saw shows the complete journey from the forest to the finished paper roll. Let us walk through each major block and understand what happens there and which piping systems are involved.

2.1 Wood Yard

At the left side of the diagram you see trees, trucks and a wood yard. Logs are unloaded here and sent to:

• Debarking drums – to remove bark
• Chippers – to cut logs into small wood chips

Piping here mainly handles:

• Hydraulic and lubrication oil for equipment
• Service water for washing chips and dust suppression

2.2 Cooking / Digesters (Compact Cooking)

The next block in the diagram is marked as Compact Cooking or digester section. Wood chips are mixed with white liquor (a mixture of sodium hydroxide and sodium sulfide) and cooked at high temperature and pressure in large vertical vessels called digesters.

Important piping systems in this section:

• High-pressure white liquor lines from chemical preparation
• Steam lines for heating the digester
• Blow lines carrying hot pulp from digester to blow tanks
• Flash steam and condensate lines

Here the temperature is high (around 140–170°C) and pressure is also high, so we normally use carbon steel or alloy steel piping and follow high-pressure design codes.

2.3 Washing & Screening

After cooking, pulp contains dissolved lignin and chemicals. It passes through a series of washers and screens. The diagram shows multiple vessels and thin lines connecting them – these represent washing stages and screening equipment.

Piping here carries:

• Pulp slurry lines between washers and screens
• Filtrate return lines for recovery
• Fresh water and wash water lines

Pressure is low but solids content is high. Lines are usually large diameter with smooth routing and fewer bends to reduce friction loss and avoid choking.

2.4 Bleaching and Chemical Preparation

To make bright white paper, the pulp must be bleached. The diagram shows a sequence of tall towers and small tanks labeled as oxygen delignification, bleaching, chemical preparation etc.

Key fluids in this area:

• Bleaching chemicals like chlorine dioxide, hydrogen peroxide, oxygen, caustic soda, etc.
• Bleached pulp slurry
• Wash water and filtrate

Because chemicals can be corrosive, piping often uses:

• Stainless steel (e.g., SS304, SS316)
• Rubber-lined carbon steel
• FRP (fiber reinforced plastic) for some low-pressure, highly corrosive services

2.5 Recovery Boiler, Evaporation & Lime Kiln

The diagram also shows a big block labeled recovery boiler, multiple evaporators and a lime kiln. This part of the plant recovers chemicals and generates energy.

• Black liquor from pulp washing is concentrated in multiple-effect evaporators.
• Concentrated black liquor is burned in the recovery boiler to produce steam and recover inorganic chemicals.
• Green liquor is converted back to white liquor using lime in the recausticizing plant and lime kiln.

Piping systems here handle:

• High-pressure steam and feedwater lines
• Black liquor and green liquor slurry at high temperature
• Lime mud slurry and flue gas ducts (not piping, but ducting)

This section is very critical from stress analysis point of view due to high temperature steam and tall boiler structures.

2.6 Paper Machine & Finishing

The long machine visible in the plant photograph is the paper machine. Pulp from stock preparation is diluted and sprayed on a moving wire mesh. Water is removed by gravity, vacuum, pressing and finally by steam-heated dryers.

Main piping for this area:

• Low-pressure pulp and white water lines
• Steam supply lines to dryer cans and condensate return lines
• Showers and spray water lines for cleaning wires and felts
• Compressed air lines for instruments and actuators

The final product is rolled into large parent rolls and sent to finishing, cutting and packaging.

3. Why Piping Design is Critical in Pulp & Paper Plants

• Handling slurry: Pulp is a mixture of fibers and water. Poor layout or sharp bends can cause sedimentation, plugging and high pressure drop.
• Corrosion: Bleaching chemicals and liquors are corrosive; wrong material can lead to leakage and unplanned shutdown.
• Temperature variation: Lines connected to digesters, boilers and evaporators undergo large thermal expansion. Proper flexibility analysis and supports are essential.
• Cleanliness: For high-quality paper, contamination must be avoided. Piping for clean water and chemicals should be well segregated and easy to flush.

A good piping engineer always studies the full process diagram first, understands which fluid flows where, and then designs the routing, supports and flexibility accordingly.

4. Fertilizer (Urea) Plant – Overview

Now let us move to the second diagram – the fertilizer plant. The photo shows tall equipment, interconnected by large-diameter pipes and platforms. The schematic drawing shows the main process blocks for manufacturing urea fertilizer.

The basic raw materials are:

• Ammonia (NH₃)
• Carbon dioxide (CO₂)

These two react at high pressure and temperature to form urea solution. After that the solution is concentrated, converted into solid granules or prills, dried, cooled and stored.

Fluids in piping of a fertilizer plant are quite aggressive – hot ammonia, carbamate solution, high-pressure steam, synthesis gas, etc. Therefore materials, corrosion allowance and stress analysis are very important.

5. Process Flow in a Urea Fertilizer Plant (Explaining the Diagram)

5.1 Urea Synthesis Section

On the left of the scheme you see a tall vessel labeled Urea Synthesis. Here, liquid ammonia and carbon dioxide react inside a high-pressure reactor.

Typical operating conditions (approximate, for understanding):

• Pressure: 130–180 bar
• Temperature: 170–200°C

Main piping systems:

• Ammonia feed lines (NH₃) – often low temperature at storage, then heated before entering reactor.
• CO₂ feed lines from CO₂ recovery/compression unit.
• High-pressure carbamate solution lines between reactor, stripper and condenser (not shown in simple diagram but present in real plant).

Because of high pressure and strong corrosive environment, materials such as special stainless steels (e.g., 25Cr-22Ni) and internal linings are used in this section.

5.2 Vaporizer / Evaporator

The next equipment in the diagram is Vaporizer. Urea solution coming from synthesis is relatively dilute. It is sent to evaporators where water is removed using steam as heating medium.

The diagram shows steam entering the vaporizer and water (condensate) leaving. Piping here includes:

• Process lines: urea solution, concentrated melt.
• Utility lines: steam supply, condensate return, cooling water.

Design focus for piping:

• Proper slope for condensate return to avoid water hammer.
• Thermal expansion of steam lines and provision of expansion loops or bellows.

5.3 Granulator

After evaporation, molten urea is sent to a granulator or prilling tower (shown as a big box in the diagram). Here the melt is sprayed and solid particles are formed.

Piping includes:

• Transfer lines for molten urea (high temperature, around 130–140°C).
• Air supply ducts/pipes for fluidization in a fluid bed granulator.
• Scrubber solution lines for off-gas cleaning.

These lines must be well insulated to prevent solidification and choking of urea.

5.4 Dryer and Cooler

The diagram shows a Dryer followed by a Cooler. Granules coming from the granulator still contain some moisture and are hot.

• In the dryer, hot air or flue gas removes moisture.
• In the cooler, ambient or conditioned air cools the granules to a safe storage temperature.

Piping/ducting for this section:

• Hot air / flue gas ducts to dryer.
• Cooling air ducts to cooler.
• Dust collection lines from cyclones and bag filters, if provided.

Though much of this is ducting, piping engineers coordinate routing, supports and interfaces with structures and equipment.

5.5 Urea Silo and Storage

Finally, the diagram shows a tall urea silo where finished product is stored. From the silo, urea is sent to bagging plant or bulk loading stations.

Piping and conveying here:

• Conveyor systems (belt, screw or pneumatic).
• Small lines for conditioning agents (anti-caking chemicals).
• Instrumentation lines – nitrogen purging, pressure/vacuum relief connections.

6. Fluids & Operating Conditions in Fertilizer Plants

Compared to a pulp & paper plant, fertilizer plants usually operate at much higher pressure and more severe corrosive conditions. Some typical services:

• Cold and hot ammonia (toxic, risk of leakage and freezing).
• Carbamate solution – very corrosive mixture of ammonium carbamate and urea.
• High-pressure steam for process heating and power generation.
• Cooling water and chilled water for condensers and coolers.

Because of these demanding conditions:

• Wall thickness of pipes is higher, with strict inspection requirements.
• Welding procedure qualification (WPS/PQR) is very important.
• Stress analysis must consider high design pressure, operating temperature and occasional load cases (e.g., relief valve reaction forces).

7. Real-Life Example: Where These Plants Are Used

• Pulp & Paper Plants: Used to produce office paper, books, newspapers, packaging boxes, tissue paper, specialty papers for food packaging, and many more products. Countries with large forests like Canada, USA, Finland, Sweden, Brazil and also India have major pulp & paper industries.
• Fertilizer Plants: Urea fertilizer from these plants is used directly in agriculture to supply nitrogen to crops. Nearly every farming country depends on such plants for food security.

So, when you see a notebook on your desk or a bag of urea in the field, remember the complex network of pipes, valves and equipment that made it possible.

8. Key Takeaways for Piping Engineers

• Always start with the process flow story. If you can explain the diagram in simple language, you are already 50% ready for good piping design.
• Identify critical services – high temperature, high pressure, corrosive or slurry lines – and give special attention to materials and flexibility.
• Understand how utilities (steam, water, air, chemicals) support the main process. Many failures happen in utility lines if they are ignored.
• Think about operation and maintenance – space for valve operation, equipment removal, and safe access for workers.

This completes your detailed introduction to Pulp & Paper Plants and Fertilizer Plants from a piping engineer’s point of view.