Steam System in petrochemical processing and Refinery

Steam System in petrochemical processing and Refinery

Steam system may be the most essential among utility systems in chemical, petrochemical and refinery plants. It is widely used in various applications. Because of its important magnitude, I decide to introduce this system applied in plant with the purpose of helping process apprentices to get the basic concept about steam application, supporting their design works as well as operation.

Purpose and Application

Thank to several advantageous properties, steam is universally employed to support many processes occurring in manufacturing. It plays an important part and is indispensable to keep the plant operating smoothly. The purpose of steam is to provide heat and power. Main applications maybe are as follows:

Steam for heat exchanger, steam for stripping, cracking steam in refinery, heating steam for reboiler or process vessel, steam for mechanical driver (e.g. turbine), steam tracing, steam air decoking of heater, steam for smokeless flare…

It is used continuously or intermittently.

Steam Distribution Level

To serve several service effectively, steam is generally divided into three levels:

-          High pressure steam: ~2.0-4.1 Mpa, gauge

-          Medium pressure steam: ~ 1.1 Mpa, gauge

-          Low pressure steam: ~0.3 Mpa, gauge

Sometimes steam system may consist of more than above levels depending on specific application.

One principle that needs to keep in mind is: using low pressure steam in heating services wherever possible.

Superheated steam

One term we must pay attention is superheated steam.

Superheated steam is steam heated to a temperature higher than the boiling point corresponding to its pressure. It is needed to distinguish with saturated steam, steam at the temperature of the boiling
point which corresponds to its pressure

Superheated steam has advantage of economical transportation so that it is used to distribute throughout plant. Frequently steam is superheated from 55 ℃to 195℃above saturation temperature in steam coils located in the inlet portion of the boilers convective section.

When necessary it is desuperheated (reduce heat) to best suite requirement as well as protect equipment from damage due to high temperature. This process is carried out through an equipment called Desuperheater.

Desuperheating is accomplished by atoming fluid and cooling fluid. Cooling fluid is employed to reduce temperature of superheated steam whereas atomizing fluid is needed when the pressure of the system is not enough to make cooling fluid atomize. 

 Fig.2 Steam Network in a petrochemical plant

Steam Network System

To fully understand about how steam system work, please refer to Fig. 2

In this network, steam system composes of three level: High, medium and low pressure steam. High pressure steam coming from boiler is desuperheated before going to high pressure-needed users. Desuperheating is implemented by boiler feed water. Steam after used become to condensate (maybe liquid or two phase). It is collected in flash drum. Flash drum has a duty to separate entrained steam and condensate. One part of high pressure steam is letdown to medium pressure steam by a control valve. Before coming to user, it is also desuperheated prior to combining with steam from flash drum. Low pressure steam is generated in the same way from medium pressure steam. Condensate from the whole system goes from the last fresh drum to condensate storage.

 

Steam Generation System

Feed sources

Steam is generated in boiler from Condensate and Boiler Feed Water.

Condensate

After steam come to users it loose heat and become condensate. Condensate is recovered whenever economically justified and returned to the deaerator as boiler feed makeup. Frequently the returned condensate must be treated to remove dissolved and insoluble iron contaminants before it is acceptable as feed.

Boiler Feed Water.

Boiler feed water is obtained from treating raw water. The purpose of treating raw water is to remove impurities and metal cations that may cause corrosion and fouling for the system.

System configuration

Please refer to Fig.3 for the system detail.

Boiler: Main equipment of the system where steam is generated. Saturated steam leaving Steam Drum is superheated in team Superheater. Chemicals are injected into Steam Drum to concentrated the dissolved solid. To maintain the boiler water solids at an acceptable concentration, a blowdown stream is continuously withdrawn from Mud Drum and Steam Drum.

Deaerator: This equipment is in charge of oxygen and carbon dioxide removal. If these entrained gases present in water feed are not completely removed in the deaerator, the resulting boiler feed water is aggressive and may cause severe corrosion of boiler feedwater pump internals at areas of turbulence or high pressure drop. Water feed is also preheated here to reduce boiler fuel consumption.

Pump: Responsible for transfer water from Deaerator to Boiler. It also creates pressure high enough for boiler requirement.

Preheater: To get efficiency and save fuel for boiler, feed is further heated to partly turn water feed into steam before being introduced into Steam Drum.

Finally, please refer to Fig 4 for overall view and detail about steam system in plant. This sample Process Flow Diagram is taken from a refinery.

Conclusion

It is difficult to mention all aspects of steam system in the framework of this article. About specific information and system design guide I will introduce in another time. However, with the concept knowledge presented here I hope that it is useful to provide direction for new process engineer who wish to get approach for system design in which steam is a system impossible to be neglected.

Fig.4 Steam System Diagram in Refinery