# Investigation of the Fluid Flow in a Large Ball Valve Designed for Natural Gas Pipelines
## 4 Hydrostatic Tests
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Test Fluid** & \multicolumn{2}{c}{**Unit**} & \multicolumn{2}{c}{**Leakage**} & \multicolumn{2}{c}{**Rate A**} & \multicolumn{2}{c}{**Rate AA**} & \multicolumn{2}{c}{**Rate B**} & \multicolumn{2}{c}{**Rate C**} & \multicolumn{2}{c}{**Rate CC**} & \multicolumn{2}{c}{**Rate D**} & \multicolumn{2}{c}{**Rate E**} & \multicolumn{2}{c}{**Rate E**} & \multicolumn{2}{c}{**Rate F**} & \multicolumn{2}{c}{**Rate G**} \\
**Rates** & & & & & & & & & & & & & \\ \hline Liquid & mm\({}^{3}\)/s & No visually & 0.006 \(\times\) DN & 0.01 \(\times\) DN & 0.03 \(\times\) DN & 0.08 \(\times\) DN & 0.1 \(\times\) DN & 0.3 \(\times\) DN & 0.39 \(\times\) DN & 1 \(\times\) DN & 2 \(\times\) DN \\ \hline Gas & Buibles/s & detectable leakage & 0.003 \(\times\) DN & 0.0046 \(\times\) DN & 0.0458 \(\times\) DN & 0.3407 \(\times\) DN & 0.4584 \(\times\) DN & 4.5837 \(\times\) DN & 7.1293 \(\times\) DN & 45.837 \(\times\) DN & 91.673 \(\times\) DN \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Maximum allowable closure test leakage rate according to API 6D [19]. The maximum allowable leakage fluid flow rate is a requirement stipulated by the beneficiary, and it is essential to achieve it. The results obtained by the hydrostatic shell test with water as the working fluid and are presented in Table 2, along with the results of the hydrostatic backseat tests (also performed with water). The tests performed with water indicated 0.043 mL/s leakage for the upstream seat and 0.41 mL/s for the downstream seat. For DN500, this means that the valve has 43 mm\({}^{3}\)/s and 41 mm\({}^{3}\)/s of fluid flow rate for leakage, respectively, and can be classified in safety category D, which accepts up to 0.1 \(\times\) 500 mm\({}^{3}\)/s of fluid flow leakage for tests performed with liquids. For tests performed with air, the limit for category D is 0.4584 \(\times\) DN500 = 230 bubbles, according to ISO 5208:2008 [29] and API 6D-paragraph 11.4.3 [19]. The hydrostatic testing of the seats with air must be performed at a pressure 10% higher than the nominal pressure. The tests performed according to the recommendations stipulated in the standards indicated that the ball valve with MM sealing and a DPE design has very good performance and can withstand 10% overpressure. The results of the backseat tests with air, presented in Table 3, classify the ball valve in the same safety category (D). The tests are mandatory for each new valve, designed to meet the special requirements of the beneficiary. If the valve does not pass the tests, another design solution must be identified, and a new valve must be constructed. It is important to point out that to obtain the safety categories AA or B, the seats should be sealed with soft gaskets. The problem with such solutions is that if a fire occurs, soft gaskets are destroyed quickly; therefore, they are not appropriate for natural gas pipelines. In conclusion, the category D identified for the investigated ball valve is accurate enough to allow the product to be used in natural gas pipelines.