These are storage tanks between units, and can serve a variety of purposes. They can dampen fluctuations in flow rate, composition or temperature. They can allow one unit to be shut down for maintenance without shutting down the entire plant. The surge drum performs various important functions. They are:
Selection of Blow down Systems: While the various systems for the disposal of voluntary or involuntary vapor or liquid are available, the actual selection of a disposal system is conducted in accordance with the expected frequency, duration of operation, required capacity and fluid properties.
Blow down System for Vapor Relief Stream: Systems for the disposal for voluntary and involuntary vapor discharges are:
- To atmosphere
- To lower pressure process vessel or system
- To closed pressure relief system and flare
- Acid gas flare
1. Vapor discharge to atmosphere: Vapor relief streams shall be vented directly to atmosphere if all of the following conditions are satisfied (as per API RP 521):
a. Such disposal is not in conflict with the present regulations concerning pollution and noise.
b. The vapor is effectively non-toxic and non-corrosive.
c. Vapor which is lighter than air or vapor of any molecular mass that is nonflammable, non-hazardous and non-condensable.
d. There is no risk of condensation of flammable or corrosive materials.
e. There is no chance of simultaneous release of liquid, apart from water.
f. Relief of flammable hydrocarbons direct to the atmosphere should be restricted to cases where it can be assured that they will be diluted with air to below the lower flammable limit. This should occur well before they can come in contact with any source of ignition.
The above condition can most easily be met if the vapors to be released have a density less than that of air. However, with proper design of the relief vent adequate dilution with air can be obtained in certain cases with higher density vapors. Methods of calculation are given in API RP 521 section 4.3.
a. Vapor from depressurizing valves shall be discharged to a closed pressure relief system.
b. Vapor which contains 1% H2S or more by volume, shall be discharged to a closed pressure relief system.
2. Vapor discharge to lower pressure process vessel or system:
Individual safety/relief valves may discharge to a lower pressure process system or vessel capable of handling the discharge.
Although this type is rarely used, it is effective for discharges that contain materials which must be recovered.
3. Vapor discharge to closed pressure relief system and flare: In all cases where the atmospheric discharge or release of vapor to a lower pressure system is not permissible or practicable, vapor shall be collected in a closed pressure relief system which terminates in a flare, namely flare system. Where the concentration of H2S is such that condensation of acid gas is probable, provision for a separate line, heat traced, shall be considered. In all cases, the installation of a closed pressure relief system shall result in a minimum of air pollution and the release of combustion products.
4. Acid gas flare: In process plants where H2S free and H2S containing streams are to be flared, consideration should be given to the installation of a separate header and flare stack assembly for the H2S containing streams. The following provisions should be studied for the acid gas flare assembly:
a. Automatic injection of fuel gas down stream of H2S pot in order to make the combustion stable.
b. Steam injection for smokeless operation shall not be considered for H2S flare tip.
c. A common pilot igniter may be used to ignite all flare stacks including the acid flare.
d. The H2S flare header and sub headers may be heat traced in order to prevent the condensation acid gas.
Blow down System for Liquid Relief Stream: Systems for the disposal of voluntary and involuntary liquid discharges are:
- To onsite liquid blow down drum.
- To lower pressure process vessel or system.
- To oily water sewers only if the material will not cause hazardous conditions.
- To pump suction if pump will not overheat or can withstand the expected temperature rise.
- To burning pit.
- To vaporizer.
Thermal expansion relief valves may discharge small quantities of volatile liquid or vapor into the atmosphere, provided the valve outlet is in a safe location.
1. Liquid discharge to onsite liquid blow down drum The liquid shall be discharged to an onsite liquid blow down drum which is capable of retaining the liquid discharged at the required liquid relief rate for a period of 20 minutes. This drum shall have a vapor discharge line to the closed pressure relief system.
2. Liquid discharge to lower pressure process vessel or system The liquid shall be discharged to a lower pressure process vessel or system which is capable of handling the required liquid relief rate plus any flashed vapor.
3. Liquid discharge to oily water sewer Liquid discharge to an oily water sewer shall be nonvolatile and nontoxic. The required liquid relief rate shall be within the oil removal capability of the oily water treating system.
4. Liquid discharge to pump suction Required liquid relief shall discharge to an upstream liquid reservoir from which the pump takes suction. The liquid relief may discharge directly to the pump suction line if sufficient cooling is provided to prevent a temperature rise of the liquid recycled through the pump when the safety/relief valve opens or when a constant displacement pump is used.
5. Liquid discharge to burning pit Liquid relief or voluntary liquid blow down which need not be returned to the process or discharged to an oily water sewer, shall be discharged to a burning pit, if environmentally accepted.
6. Liquid discharge to vaporizer The liquid shall be discharged to a vaporizer which is capable of vaporizing a liquid relief of no more than 5,000 kg/h.
Design of Disposal System Components: Depending on the process plant under consideration, a disposal system could consists of a combination of the following items: piping, knock-out drum, quench drum, seal drum, flare stack, ignition system, flare tip, and burning pit.