Shell and tube heat exchanger issues 1

[365che]

Hello everyone. First time posting here.

I'm looking for some opinions and recommended courses of actions regarding two issues we are facing with shell and tube heat exchangers we run in series for our EO reactor heating/cooling needs. These HXs both utilize cooling and heating through the shell side, thermocycling 10-20k+ times a year. We run batch processes.

We run over 30+ reactions in, lets call it Reactor A, utilizing EO and usually some sort of fatty acid. The reactions are all exothermic. We try to maintain a temperature setpoint through the use of two shells and tube heat exchangers in series.

The flow is... reactor A -->HX 1-->HX 2--> back into reactor.

The temp setpoint differs from product to product. The product goes through the tube side, and utility goes through the shell side on both heat exchangers. We utilize 115 psi steam for heating, and cooling tower water for cooling which ranges from (5C-40C) depending on time of year. The utility feed, whether steam or cooling water, into both heat exchanger shells are in parallel.

The reactions are exothermic but sometimes we over cool and need to utilize heating. Therefore in a typical batch we may thermocycle anywhere from 10-20 times. There are three states in the programmed logic. COOL, OFF, HEAT. Therefore there are 4 transitions. COOL TO OFF. HEAT to OFF. OFF to COOL. OFF to HEAT. If the setpoint is 160 C, there is a +/- 2.5 C range from which the system will be in OFF state.

Depending on if we are heating, cooling, or off... certain valves will be opened and closed.

See attached photo. This shows one of the HXs while it happens to be in the HEAT state.

I will outline all three states below.

KEY:

Steam
CWS (Cooling water supply)
CWR (Cooling water Return)
CND (Condensate return)

1) When in the OFF position the CWR valve is open and all others are closed
2) When in the HEAT position (as seen in the attached photo) the steam valves are open, the CND valve is open, all others are closed.
3) When in the COOL position the CWS is open and CWR is open, all others are closed.



Our first issue is WATERHAMMER. This appears to occur when we go from HEAT to OFF. When we go from HEAT to OFF the steam and CND valve close simultaneously, and a millisecond later the CWR valve open since that is the default OFF state as mentioned above. It's at this moment that we witness waterhammer through the CWR line. It is vicious. One theory is that the steam/condensate hasn't evacuated the shell of the HX + is under vacuum from turning the heat off, and once the CWR valve is opened you are introducing the steam/condensate to the cold water in the CWR line which creates thermal shock water hammer.

Our second issue, which is likely related to the waterhammer described above, is damage to the tubes and tube sheet.

[highlight #729FCF]My questions/Suggested corrective actions would be the following.[/highlight]

1) Would it make sense to delay the closing of the CND return line and delay the opening of the CWR line when transitioning from HEAT state to OFF state? In other words, instead of the steam and cnd valve closing simultaneously, we would have the steam valve close first once high temp set point is reached, then leave the CND valve open to let it drain for 10-15 seconds, then close the CND line, then open the CWR line.
2) Should we have a vacuum breaker on the CWR/CND common line that exits the HX. Perhaps placed next to the TT on the picture I've attached. I've determined that we have no vacuum breaker on our system. Could we be creating vacuum when we go from HEAT to OFF and holding condensate in the shell, which then allows for waterhammer to occur after opening the CWR line. Could this be what is lifitng our PSV every 2-3 months? Could this be what's causing damage to our tubes, and tube sheet?
3) Since we use steam through this shell, should we have an impingement plate on the utility inlet?

I was thrown this project/investigation not to long ago, but this waterhammer has apparently been an issue on these HXs for decades here. No one has been able to implement a solution which works.

Are my suggested courses of actions reasonable? Are there any other recommendations?

Thank you

365CHE

 

[365che]

georgeverghese,

I agree with you that this scheme was terribly designed from the start. It's been this way for at least a decade from what I've been told. The reaction pressure is usually fairly low around 35 psig. There usually is a momentary 50 psig spike for each batch. Our steam and cooling water pressures are higher in the shell side than the EO moving through the tube side. When there is failure at the tubes or tube/tube sheet interface, we usually see a spike in the hydroxyl levels in our product upon quality checks, indicating that water has entered into the TUBE side. Additionally, we load the reactors with an excess of fatty acid prior to the slow addition of EO. The EO reacts almost immediately upon entry to the reactor.

I also pitched the idea of two separate parallel banks of HXs. They want to try simpler solutions before spending any significant amount of capital.

 

[365che]

Snickster,

See attached P&ID.

 

[georgeverghese]

Presume that TIT on the product line downstream of HX A is the one providing feedback to the temp control loop that operates the 3way valve upstream of HX B. That location for the TIT does seem awkward - wouldnt it be better if it were upstream of the HX A to stabilise the reactor temp and reduce these wild swings in temp? Also check if this TIT is a thermocouple (not an RTD) sitting in a thin wall thermowell to reduce response lag.
Recirculation pumping rate also has an effect on temp stability; the larger the recirculation flow, the more stable the reactor temp would be.

 

[365che]

George,

There actually is a temp transmitter on the bottom of the reactor (not shown in the drawing). That temp transmitter controls whether HEAT, OFF, or COOL is activated for both HX A and B. The three way valve downstream of HX A is not involved in the logic. If they want to skip HX B, they can activate the 3-way and go REACTOR --> HX A --> REACTOR --> REPEAT

I Don't believe the TIT you are referring to is involved with the logic.