PTQ Q1 2023 Issue

Motive nozzle moved forward

Diuser

Diuser throat

Discharge

Motive steam

Steam chest

Suction load

Figure 9 Motive steam nozzle moved forward

Certified performance curves When new ejectors are purchased, the end user should require manufacturers to supply certified performance curves similar to Figures 7 and 8 based on shop testing. If the actual ejector is too large to test, which is often the case for the first stage, the test should be based on a rep - lica ejector that is scaled down in size. 2 Would you buy a pump without a proper performance test? When an ejec- tor is not thoroughly tested, it is common for it to not per- form per the supplied performance curve, including not meeting the stated MDP. The first-stage ejector’s MDP can be determined in the field by throttling the second- stage ejector suction isolation valve while simultaneously measuring first-stage suction and discharge pressures. When discharge pressure changes begin to increase suc- tion pressure, the true first-stage MDP is measured, irre - spective of what may be listed on the performance curve. As the ejector begins to ‘break’, the suction pressure becomes unstable and noise increases as the fluid flow direction reverses. Flow reversal or ‘backfiring’ makes a very distinct sound with often an audible noise increase at the ejector discharge just prior to first-stage suction pres - sure instability. In some instances, suction pressure has jumped by 20-30 mmHg or more almost immediately with extreme noise levels. This method has shown, in several instances, the actual MDP to be much lower than antici- pated based on design information. In field tests where the authors have verified that steam flow rate and temperature match design, some systems performed per vendor-provided performance curves while others did not. In several instances, field tests proved actual MDP as much as 15 mmHg below the vendor-stated value. Several refiners have had to move the motive steam nozzle forward in the steam chest to increase MDP after ejector systems were installed. Moving the steam nozzle forward in the steam chest (see Figure 9 ) to increase MDP changes the performance curve and reduces the suction capac- ity for a given pressure. Other times, motive steam nozzle changes and larger second-stage ejectors circumvent first- stage inter-condenser design flaws, but first-stage suc - tion pressure also increases. Unfortunately, sometimes the vacuum system performance is so bad that complete sys- tem replacement is needed. In order to avoid performance break, it is imperative that ejectors are shop tested so that an accurate performance curve can be generated and the true MDP is verified.

First-stage inter-condenser operating profiles Modern condensers use an X-shell with long air baffles. The baffle provides a separate zone to sub-cool the out - let vapour below the bulk condensate. This sub-cooling minimises second-stage process load by routing the outlet vapour across the exchanger tubes with the coldest CW temperature. When the condensate leaving the X-shell is lower than the vapour outlet, as shown in Figure 4, there is bypass and the second-stage load is higher than it should be, resulting in higher second-stage suction pressure and higher first-stage ejector discharge pressure. The pressure and temperature profile for the first-stage ejector and condenser designed from the Table 1 loads are shown in Figure 10 . The first-stage ejector discharge pressure of 87 mmHg absolute is based on the second- stage ejector suction pressure of 75 mmHg absolute plus pressure drop allowance for piping and the first-stage con - denser. The MDP for this particular ejector is 98 mmHg. This implies an 11 mmHg margin for MDP above normal operating discharge pressure. The first-stage inter-condenser design duty is 65 MM Btu/hr. The CW inlet and outlet temperatures are 90°F and 103°F, respectively. The outlet gas stream flow rate was based on 100°F gas temperature at 80 mmHg absolute pressure. The design condensate outlet temperature is 105°F. For first-stage inter-condensers, it is not unusual for

Vacuum column overhead

1st stage ejector

1st stage inter-condenser

100

105

Pressure, mmHgA Temperature, ˚F

2nd stage ejector

Condensate

Figure 10 Ejector design conditions

PTQ Q1 2023