View source for Amine Surge Tank(SV-1101)

As described in [[Amine Regeneration]], the lean amine leaves the lean/rich exchangers through the regenerator LCV, and flows to the amine surge tank.

=Purpose=

The amine surge tank is a gas-blanketed, atmospheric pressure storage tank that stores the process train’s warm, lean amine.

The purpose of the Amine Surge Tank is to provide amine storage capacity for the amine booster pumps to draw from and when the process train is temporarily offline. It also provides a buffering ability to:
*supply an increased demand for amine as raw gas load increases
*store an increased volume of amine as raw gas load decreases

The amine surge tank is blanketed with fuel gas so that the warm lean amine is stored under oxygen-free conditions to prevent amine degradation (refer to [[Amine Solution Conditioning]]).

[[File:AmineSurgeTank.png|center|frame|Surge Tank Cross-sectional View]]

===Offline Storage Capacity===

The Amine Surge Tank has the storage capacity to hold amine when the train is temporarily offline. However, the surge tank is not large enough to hold the train’s entire volume of amine (such as during a turnaround shutdown when all the train’s vessels/piping are drained of amine).

<u>'''Note:'''</u> Because the Amine Surge Tank is unable to hold the total volume of amine during turnaround-type shutdowns, additional storage tanks are available (described in [[Amine Solution Conditioning]].)

===Changes in Tank Level===

During steady raw gas flow to the A and B contactors, the amine circulation flowrate to each train is steady. The amine surge tank level is constant because the amine flowrate entering the surge tank equals the flowrate leaving the surge tank. The surge tank’s level is also typically constant if the amine water content is constant.

The level in the surge tank may change because of the following:
*water content of amine
*amine solution held up on a tower’s trays and in the downcomers (foaming)
*a setpoint change to one of the amine vessels
*equipment leaking to the floor (or outside)
*equipment passing to the amine drain vessel (as confirmed by the CRO trending the level of the amine drain vessel)

=Operation=

The Amine Surge Tank provides lean amine storage and is equipped with the following inlet/recirculation flowlines:
*the partially cooled lean amine from the lean/rich exchangers (that enters via the regenerator level control valve - LV-1816)
*a filtered lean amine from the Lean Amine Carbons fines filter returns to the tank via a normally open (NO) block valve (Module 9—-Amine Solution Conditioning describes the charcoal tower)
*a booster pump recirculation line from the two booster pumps. The line is used by the Area Operator when switching the booster pumps.
*a charge pump recirculation line from the charge pumps. The line is used by the Area Operator when switching the charge pumps.
*a makeup amine inlet line from the amine storage tank and make up pump enters the tank through a manual block valve

The amine surge tank is equipped with these additional lines:
*partially cooled lean amine is drawn out of the surge tank via a bottom outlet flowline equipped with a NO block valve. (The outlet flowline directs the lean amine to the suction of the online booster pump)
*Although the Amine Surge Tank stores warm lean amine, the tank is equipped with a Glycol heating coil supplied from the HM system. The coil is normally blocked in because amine heating is only required during winter shutdown. When the coil is valved in service, glycol flows through the coil provides local heating to keep the tank warm and maintain the amine’s temperature above freezing. The amine in the tank itself must not be overcooled because overcooled amine becomes very viscous and difficult to pump. As amine cools below 50ºF, its viscosity greatly increases; at 28ºF, the amine freezes. The Glycol heats the amine, and then returns to the HM return header.

=Process Controls and Instrumentation=

==Pressure==

In the amine surge tank, a change in the tank’s vapor space volume directly affects the tank pressure:
*an increase in the amine level decreases the tank’s vapor space volume
*a decrease in the amine level increases the tank’s vapor space volume
*ambient cooling of the tank’s warm amine contents resulting in contraction of the liquid, which increases the vapor space volume in the tank

Because of the possible changes in tank vapor space volume (and tank pressure), the Amine Surge Tank must be able to pressurize or vent in order to operate within its precise pressure range (and thereby avoid vacuum collapse or overpressure rupture).


Pressure in the amine surge tank is controlled as follows:
*two blanket fuel gas pressure regulators maintain a slight positive pressure on the tank; the blanket gas ensures that the tank vapor space remains free of oxygen (to prevent oxygen’s degradation of the stored amine). The blanket gas regulators are as follows:
:-the upstream regulator reduces the pressure of the fuel gas to 1.0 in. WC, the blanket gas then enters the tank
:-The downstream regulator is set to open if the pressure exceeds 3.0 in. WC; discharging the gas to the LP Flare
:-a pressure vacuum relief valve (PVRV-1947) is located on a drawoff line that leads from the blanket gas then enters the tank


The PVRV is a specialty valve that is spring loaded to vent overpressure/relieve vacuum as follows:
*the PVRV vents pressure if the pressure in the surge tank rises above a gauge pressure of 4.5 in. WC. The PVRV relieves to the a safe location (Atmosphere)
*the PVRV draws in air to relieve tank vacuum if the pressure in the tank drops to more than 2 in. WC below ambient atmospheric pressure.


A thief hatch which is a specialty roof hatch that is spring loaded to vent overpressure/relief vacuum as follows:
*the thief hatch vents pressure to atmosphere if the pressure in the tank rises above a gauge pressure of 15.5oz/-in2. WC


:''The opening of the PVRV to relieve vacuum is abnormal because the Amine Surge Tank’s upstream regulator should be maintaining positive blanket gas pressure on the tank (and thereby preventing tank vacuum conditions). The opening of the PVRV to relieve tank vacuum indicates a problem with the tank’s regulated blanket gas supply. Although the opening of the PVRV to relieve tank vacuum protects the tank from a vacuum implosion, the PVRV’s opening creates another problem. If air is drawn into the tank, the tank’s amine is at great risk of oxygen-based degradation.''

==Pressure Instrumentation==

The Amine Surge tank is equipped the following temperature instrumentation:
*one gauge (PI-1954A) shows the pressure of the upstream fuel gas regulator
*a pressure transmitter (PIT-1954) relays surge tank pressure to DCS for onscreen indication


==Temperature==

The Amine Surge tank is equipped with the following temperature instrumentation:
*a local temperature gauge (TI-1949)
*a temperature transmitter (TIT-1950) that relays surge tank the following temperature instrumentation to DCS for onscreen indication

==Level==

The Amine Surge tank is equipped with the following level instrumentation:
*a local level Gauge (LI-1953A) shows the amine level in the tank
*a level transmitter (LIT-1951) provides onscreen tank level indication at DCS and to a digital readout at ground level in the Process building


=Alarms and Shutdowns=

===Level===

At the amine surge tank, abnormal levels cause the following alarms and shutdowns to appear onscreen at the DCS:
*two level transmitters (LIT- 1951) and (LIT-1953)
*low alarm (LAL-1951) is triggered at 20% level, a low level shutdown (LALL-1953) is activated at 14.5%
*a high-high alarm (LAH-1951) is triggered at 84% level. A high level shutdown is activated at 95%


===Temperature===

*A low temperature alarm (TAL-1950) alarms in at 120 degrees F
*A high temperature alarm (TAH-1950) alarms in at 175 degrees F

The glycol heating coil also has a local flow indicator (FO-1955) downstream of the manual globe valve, which gives the area operator local flow indication.


=Operations Monitoring=

During monitoring rounds at the Amine Surge Tank, the Area Operator:
*ensures that gas blanketing is maintaining the tank pressure at 1 to 3 in. WC
*checks that the tank’s amine level is at the normal 50% operating level
*verifies that the heating coil is in service if need be
*verifies the amine surge tank common suction piping configuration.
*Checks the tank temperature

The Area Operator must be ready to respond to prompt CRO requests about investigating a rapidly dropping surge tank level. The surge tank level may be dropping for the reasons described in [[#Purpose]].

In case of a train shutdown, the regenerator trays and downcomers fully drain; the draining amine may accumulate in the amine surge tank due to the open regenerator LCV and cause the tank level to rise greatly. As a result, the Area Operator must be ready to respond to the prompt CRO request to block in the amine return to the surge tank. This Area Operator task involves blocking in the surge return inlet manual valving. Blocking in the valve stops the regenerator amine drainage and thereby stops the level increase in the amine surge tank.


:<u>'''Important:'''</u> Promptly blocking in the amine return to the Amine Surge Tank is important to prevent amine flow from the Regenerator from:
:*overfilling the surge tank
:*directing very hot amine solution flow through the exchangers without proper heat transfer. The overheating will cause severe hammering to the exchangers and piping.