PTQ Q1 2024 Issue

The hot feed, initially at 125°C, is preheated to 218°C before entering the FSD. In situations where there is a fail- ure in the hot feed supply, a cold feed is used as a replace- ment. This cold feed, starting at 40°C, is then preheated to 155°C before being introduced to the FSD. It is assumed that the FSD has a similar dimension to the lean amine surge drum of Project A above and the FSD is operating at the same pressure of 2.5 bara. From this, the following is calculated:

In the case of determining the nitrogen blanketing flow rate for tanks, the API 2000 method serves as a standard- ised procedure. Estimation of inbreathing due to liquid transfer effect According to Section 3.3.2.2.2 ‘Inbreathing’ of API 2000, the estimation of inbreathing, resulting from the maximum outflow of liquid from a tank, necessitates consideration of the rate of volume change of tank vapour space due to liq- uid movement. The requirement for inbreathing, denoted by V inL and measured in normal cubic meters per hour of air, should equal the maximum liquid discharging capacity for the tank, denoted by V liq and measured in cubic meters per hour. This correlation is presented in Equation 1 . Here, V liq signifies the rated capacity of the pump connected to the tank:

= 78 m³ = 53 m³.

V Total V NLL ϱ hot ϱ cold

= 700 kg/m³ = 751 kg/m³

P normal = 2.5 bara P final

= To estimate (bara)

Liquid volume change (dV) = 53 X (1- 700/751) = 3.6 m³ Vapour volume above NLL (V vapour ) = 78 – 53 = 25 m³ P final = 2.5 X (25/(25+3.6) = 2.2 bara From the above calculation, it is evident that the pressure reduction due to liquid volumetric contraction when replac- ing hot feed with cold feed is relatively minor. Determination of nitrogen requirement for tank blanketing Inert gas systems, like those using nitrogen, help prevent air from entering a tank when there is a chance of vacuum generation inside it. These systems make tanks safer by reducing the chance of creating explosive atmospheres and minimising the risk of dangerous flashbacks. However, it is important to note that this nitrogen blan- keting system should not replace vacuum relief devices. Despite having an inert gas system in place, the vacuum relief devices need to be large enough to handle situations where the inert gas might not be available. Vacuum conditions in a tank can occur due to two main causes:  Liquid transfer effect: This phenomenon occurs when there is an outflow of liquid from the tank without a corre - sponding inflow, creating a vacuum.  Thermal effect : Changes in atmospheric conditions, such as a drop in temperature or shifts in weather patterns (like wind changes or precipitation), can lead to the con- traction or condensation of vapours, consequently resulting in a vacuum.

V inL = V liq

Eq. 1

Estimation of inbreathing due to thermal effect The API-2000 standard, specifically Section 3.3.2.3.3 ‘Thermal Inbreathing,’ provides guidelines for calculating inbreathing attributed to thermal effects. As per this section, the thermal inbreathing of the tank, measured in normal cubic meters per hour of air, is calculated in line with Equation 2 :

V inT = C X V tk

0.7 X Ri

Eq. 2

Where: V inT is inbreathing flow rate (Nm³/h)

C is a factor that depends on vapour pressure, average storage temperature, and latitude (ref API 2000 Table 2 ) V tk , which represents the tank’s vapour volume, is expressed in cubic meters. For vertical cylindrical tanks, it is accept- able to calculate this volume based on the tank shell height, not including the tank roof. The term Ri refers to the reduction factor for insulation. In situations where no insulation is used, such as this, Ri is assigned a value of 1. When establishing inbreathing requirements, the design basis must account for the most significant single contin - gency or any plausible combination of contingencies. To ensure comprehensive coverage, the total normal inbreath- ing of the tank should, at a minimum, consider the combined effects of liquid transfer and thermal conditions during normal operations. As a result, the normal inbreathing

C factors

Latitude

C factor for various conditions

Vapour pressure similar to hexane

Vapour pressure higher than hexane, or unknown

Average storage temperature, ºC

<25

≥ 25

<25

≥ 25

Below 42°

4 3

6.5

Between 42° and 58°

5 4

Above 58°

2.5

Table 2

PTQ Q1 2024