[maxim]

Dear colleges,
In application I face with, the export On-Spec. oil is being transfered from the originating plant by reciprocating pumps (1st. stage) through 130km pipeline. Export oil comes to the intermediate site, where surge (buffer) pressurized vessels are installed (mandatory client request) and oil from these vessels is being pumped to the purchaser by reciprocating pumps (2nd. stage).

Positive pressure at the suction of 2nd. stage pumps is being pertained by Nitrogen bllnketing of surge bullets, between 2.4barg & 3.6barg, thus eleminating need for 2nd. stage booster oil pumps.
This is client mandatory request due to banned oil venting and available surge (buffer) bullets installation.

My task is to design PSA Nitrogen Generator and size pure nitrogen receiver.

There are 5 (five) Surge (buffer) bullets, each of 200m3 volume. These bullets MAWP is 25barg (LPG bullets), and inert gas (N2) positive pressure is maintained by PRV set at 2.4barg and backpressure control valve set at 3.6barg (vents to atmosphere through vent stack). Switching from one to the next bullet is automatic based on LL and HH levels.

Under the pressure which will exist in surge bullets, there won't be any vaporization of Spec. oil at whole temperature ranges.

All bullets work in cycle, meaning:
- During 1st. hour 1st bullet is being filled for an hour,
- During 2nd.hour oil in 1st bullet is being settled, while 2nd. bullet is being filled for an hour,
- During 3rd.hour sample oil is taken from 1st bullet is being settled, while 3rd. has started to be filled,
- During the 4th. Hour, oil from 1st. bullet is being unloaded (can be unloaded for app. 48minutes, without ba-pass mode)
- During the 5th. Hour, 1st tank is empty, while 5th. Bullet is started to be filled

Capacity of 1st. stage reciprocating pumps is 150m3/hr, while maximum capacity of 2nd. stage reciprocating pumps is 200m3/hr, meaning that outflow overcomes inflow.
Bullets are being filled to 80% volume and above them is pressurized N2 blanket (min. 2.4barg). All bullets have vapor spaces connected.

Blanketing requirement is usaual equal to maximum output flow from the bullets, with no incoming flow in them, what would give for max. outflow of 200m3/hr, N2 required blanketing quantity of app. 2.4 x 200 Sm3/hr + 200 Sm3/hr = 680 Sm3/hr.

However, during normal operating case the N2 required blanketing quantity is significanly lower and equals app. (200 – 150) x 2.4 + 50 = 170 Sm3/hr.

As N2 blanket pressure is being increased to max. 3.6barg with liquid inflow, there is certain N2 excess capacity in the bullets vapor space (app. 20% when filled) and in empty bullet.

As in the case with no inlet flow, there is no process requirement to pump all spec oil out from all the bullets, it can be done if required with half of deign flow for double time (decrease required N2 pressure), or if max. output pumping is continued and N2 blanket pressure decrease, the 2nd. stage reciprocating pumps will anyhow be stopped due to PSLL action.

If adopt Nitrogen Generator capacity of 1.2 x 200 Sm3/hr = 240 Sm3/hr @ usual 5.5bar (pressure that N2 Receiver is kept with), the normal operating case is covered and there is no jeopardy of harming the bullets.

Please, can you give your thoughts on this issue and give me recommendation for sizing premises.

Is it better to go for higher Nitrogen capacity or higher outlet pressure from N2 Generator or increase N2 Receiver.

Shall I comply with capacity in normal operating case (240 Sm3/hr) or max. outflow (680 Sm3/hr).

Many thanks