Control valve hydraulics is a key step in specifying a control valve in Process industry. A proper and comprehensive hydraulics involves not only determining the optimum pressure drop across the control valve for maximum, minimum and normal flow rates, but also checking for the possibility of cavitation in the valve and deciding the type of the valve. It can also help in selecting the flow characteristics type of the valve. If the pressure drop is too high, it means the loss of energy. On the other hand, if pressure drop is too low, it could compromise the controllability of the control valve. Hence, an optimum pressure drop should be determined.

A control valve’s controllability can be considered acceptable if it’s opening at maximum flow rate does not exceed 80-90% of full opening, and the opening at minimum flow does not fall below 10-20% of the full opening. If the pressure drop is too low, the opening at maximum flow may exceed the said range and at minimum flow might fall below the said range leading to poor controllability of the control valve.

Thus, the optimum pressure drop across a control valve involves a trade-off between the cost of energy and the controllability of the valve. This optimum pressure drop is a certain percentage of the total pressure drop across the control valve circuit (including control valves, pipes, fittings, any equipments and instruments) depending upon the range between the maximum flow and minimum flow which control valve has to handle.

In most of the common cases, a control valve handles maximum flow and minimum flow at 110% and 50% of the normal flow as plant turn-down is generally 50% and 10% margin over normal flow is the common consideration. In such cases, a control valve pressure drop of 25-30% of total dynamic losses across the control valve circuit will produce reasonably good controllability of the valve. This pressure drop would increase or decrease if the range between maximum flow and normal flow increases or decreases.

The controllability of a control valve can be evaluated using flow coefficient of the control valve. Flow coefficient (Cv) is defined as volumetric flow rate in gallons per minute of water through the valve at 60 deg F when pressure drop across the valve is one psi. Cv can be calculated using formula given in the standard ISA-75.01.01-2007.

To achieve the desired controllability, the ratio of maximum flow coefficient (Cv) to minimum Cv should preferably be not more than 15. If this ratio far exceeds the said value, the travel at minimum flow may be below 10% of the rated travel or the travel at maximum flow may exceed 90% of the rated travel, which means poor controllability. In that event, pressure drop across the control valve should be increased so that the said ratio can be lowered.

A comprehensive and thorough hydraulics should also involve preliminary size check of the control valve. Though the size of the control valve is determined by the control valve vendor, it’s worthwhile to do the preliminary check of the valve size during design phase itself to avoid any possibility of size becoming abnormal i.e. either higher than the pipe size or too less than the pipe size during the later phase of the engineering.

This can be done by estimating the rated Cv of the valve. For this Cv, given type of the valve and given pressure rating, control valve size can be obtained by referring to a control valve manufacturer catalogue. If this size is higher than the pipe size, the pressure drop across the control valve should be increased. On the other hand, if the estimated size of the control valve is too less than the pipe size, the pressure drop across the control valve should be lowered without compromising its controllability. This step may also help the selection of the right type of the valve. For example, changing the globe valve to butterfly valve or vice versa could yield proper size as well as controllability of the valve.

Control valve hydraulics should also check for the possibility of cavitation in the valve. During the flow through a control valve, the minimum pressure occurs at the vena contracta and then pressure increases along the path of flow till the outlet of the control valve. Vena contracta is a point in the flow path where flow area is the minimum. Therefore, velocity is the maximum and, hence, pressure is the minimum at vena contracta. For a liquid flow, if the pressure at the vena contracta is less than vapor pressure of the liquid, vapor bubbles are formed. As pressure recovery takes place downstream the vena contracta, the vapor condenses and the bubbles collapse. As bubbles collapse, it causes impact on the valve body and may cause erosion, noise and vibration. This phenomenon is called cavitation.

Full cavitation occurs when pressure drop across the control valve is more than or equal to certain minimum pressure drop (or critical pressure drop) and the pressure at the outlet of the control valve is more than the vapor pressure of the liquid.

A Process engineer should try to avoid the possibility of cavitation while specifying the control valve. Possibility of the cavitation can sometimes be avoided by making certain adjustments during hydraulics. For example, pressure drop across the valve can be fixed in such a way that it is less than the critical pressure drop, off-course without compromising the controllability of the valve.

Control valve hydraulics can sometimes also help in deciding the flow characteristics of the valve. Most common types of inherent flow characteristics are linear, equal percentage and quick opening. Pressure drop is one of the factors that help determine the flow characteristics type. Linear characteristic should be specified if most of the pressure drop as proportion of total pressure drop in the system is across the valve itself so that pressure drop across the valve remains nearly constant for varying flow rates. Equal percentage characteristic should be specified where high proportion of the pressure drop is in the system other than the valve i.e. in pipes, fittings, equipments etc.

Date of Publication: 16 September, 2017

About Author: Satyendra Kumar Singh, B.Tech. (Chemical Technology) + M.B.A., is proprietor of Satsha Management Services-an award winning design engineering and management consulting company (www.satshamanagement.com). He possesses approximately 30 years’ experience in engineering consultancy in process and energy industries. Satyendra has authored several papers on energy, business and management, which have been published in some renowned journals/magazines such as ‘Chemical Engineering’, ‘Process Worldwide’, ‘Modern Manufacturing India’. He may be reached at satyendra.singh@satshamanagement.com, Ph. +919811293605.

Pressure safety valves (PSVs) are one of the most commonly used devices in Process industry to protect a system from overpressure caused by any upset condition. They are one of the most reliable devices to reduce the risk arising out of over-pressurization. Typically, they can reduce the risk by 100 times. Therefore, it is extremely important to specify them correctly. Here are the step-by-step guides to specify a PSV:

i) Study the system to be protected against overpressure.

ii)  Identify various upset scenarios which can cause excessive pressure in the system i.e. which can cause a pressure which is more than design pressure of the system.

iii)  Such overpressure scenarios could be blocked outlet, control valve failure, external fire, instrument air failure, thermal expansion, heat exchanger tube rupture, cooling water failure, air-cooler fan failure, column reflux failure, etc.

iv) Define set pressure, over pressure and relieving pressure of the PSV depending upon maximum allowable working pressure and accumulation as per API Standard 520 Part I, and ASME Section VIII Division I or ASME Section I as the case may be. For this purpose, one must have clear understanding of difference between accumulation and overpressure. If the set pressure is less than maximum allowable working pressure (MAWP), the overpressure could be more than accumulation. However, if PSV set pressure is same as MAWP, the accumulation and overpressure cannot exceed the accumulation. The relieving pressure would be set pressure plus overpressure.

v)  Thus, if MAWP is 20 bar (g) and set pressure is 19 bar (g), for single operating PSV (1×100% or 2×100%) and for non-fire and non-steam case, the maximum accumulation would be 10% of MAWP or 2 bar, whereas the maximum over pressure would be 15.8% of set pressure or 3 bar. On the other hand, if set pressure is same as MAWP i.e. 20 bar (g), the maximum overpressure would be equal to accumulation i.e. 2 bar or 10% of set pressure. The maximum relieving pressure would be 22 bar (g). Design pressure can be considered in place of MAWP for this purpose as design pressure does not exceed MAWP.

vi)  While set pressure cannot exceed the maximum allowable working pressure (MAWP) for single operating PSV, it can exceed the MAWP for one or more PSVs in case of multiple operating PSVs (e.g. 2×50% or 3×50%). Thus, in above example, maximum set pressure can be 20 bar (g) for single operating PSV. Whereas for multiple operating PSVs, the maximum set pressure of the first one would be 20 bar (g) and of other operating PSVs 21 bar (g).

vii)  Overpressure should be taken differently for fire and non-fire cases and must be defined carefully for single and multiple operating PSVs. Thus, in above example, for single operating PSV (1×100% or 2×100%), with set pressure as 20 bar (g), the maximum overpressure would be 2 bar or 10% of set pressure for non-fire case and 4.2 bar or 21% of set pressure for fire case. In case of multiple operating PSVs, the maximum overpressure of the first PSV would be 16% of set pressure or 3.2 bar and of other operating PSVs, 10.5% of set pressure or 2.2 bar for non-fire case. For fire case, the maximum over pressure would be 4.2 bar or 21% of set pressure for first PSV, and 3.2 bar or 15.2% of set pressure for other operating PSVs.

viii)  The set pressure, accumulation, over pressure and relieving pressure for steam systems for single and multiple operating PSVs must be defined differently and as per ASME Section I and/or IBR (for India only).

ix)  Find out the relieving rate of the PSV corresponding to each of the applicable upset scenarios. Reliving rate calculation must be done at relieving conditions.

x)  Calculate the back pressure (superimposed plus built-up) depending upon the PSV discharge destination (flare or atmosphere in general).

xi)  Select the type of the PSV appropriately. One of the criteria for the selection is the built-up backpressure as proportion of the set pressure of the PSV. There could be a trade-off between cost of PSV (depending upon its type) and discharge pipe size, especially in the cases where PSV set pressure is low and optimum selection of PSV type should be done considering this trade-off.

xii)  Specify relieving rate, PSV design parameters and properties for all the applicable scenarios in Process datasheet of the PSV.

Date of Publication: 7 August, 2017

About Author: Satyendra Kumar Singh, B.Tech. (Chemical Technology) + M.B.A., is proprietor of Satsha Management Services-an award winning design engineering and management consulting company (www.satshamanagement.com). He possesses approximately 30 years’ experience in engineering consultancy in process and energy industries. Satyendra has authored several papers on energy, business and management, which have been published in some renowned journals/magazines such as ‘Chemical Engineering’, ‘Process Worldwide’, ‘Modern Manufacturing India’. He may be reached at satyendra.singh@satshamanagement.com, Ph. +919811293605.

The Indian petroleum refining industry comprises of private, public and joint venture refining companies.  Of the total installed capacity of 234 million metric tonnes per annum (MMTPA), 34.1 per cent comes from private sector, 6.4 per cent from joint venture and 59.5 per cent from public sector.

All the public sector refineries are owned by state-run oil majors Indian Oil Corporation Limited (IOCL), Bharat Petroleum Corporation Limited (BPCL), Hindustan Petroleum Corporation Ltd (HPCL) and Oil and Natural Gas Corporation Ltd (ONGC). Reliance Industries Ltd (RIL) and Essar Oil Ltd (EOL) are the private sector refiners. The two joint venture refining companies are HPCL Mittal Energy Limited (HMEL) and Bharat Oman Refineries Limited (BORL).

Indian refineries are quite diverse in respect of their capacity. On one hand, there are small refineries such as ONGC, Tatipaka with a capacity of 0.066 MMTPA and IOCL, Digboi having a capacity of 0.65 MMTPA. On the other hand, there are big refineries such as EOL, Vadinar with a capacity of 20 MMTPA; RIL, Special Economic Zone (SEZ), Jamnagar of 27 MMTPA; and RIL, Jamnagar of 33 MMTPA. RIL refineries are amongst some of the biggest refineries of the world.

Overall Product-mix

Refined petroleum products can broadly be divided into three categories-light distillates, middle distillates and heavy ends. Light distillates include Liquefied Petroleum Gas (LPG), Naphtha, and Motor Spirit (MS) also called Gasoline or Petrol. Middle Distillates include Kerosene, Aviation Turbine Fuel (ATF), and High SpeedDiesel (HSD)also called Diesel or Gas Oil. Heavy ends include Lube Oil, Furnace Oil (FO), Low Sulphur Heavy Stock (LSHS), Bitumen, Petcoke, etc.

Based on production of the petroleum products from all the Indian refineries for 2015-16, the overall yield of light distillates and middle distillates for all Indian refineries is 27.6% and 51.2% respectively. The low value FO/LSHS yield is 4.7%. The low yield of FO/LSHS is beneficial as it is a low margin product.

Product-mix varies significantly from refinery to refinery.RIL and EOL refineries have no Fuel Oil(Furnace Oil or Low Sulphur Heavy Stock) in their product-mix.Whereas, most of the fuel oil is produced by public sector refineries, mainly the older and un-modernizedones.

The yield of light and middle distillates has been increasing and that of FO/LSHS has been declining over the years, which shows that Indian refiners have been investing in the upgradation and modernization of their refineries.

Market Share of Various Refiners

Most of the petroleum products are marketed in domestic market by three state-owned refiners or oil marketing companies (OMCs) viz. IOCL, BPCL and HPCL, and two private refiners RIL and EOL. Other public sector companies such as GAIL India Limited, ONGC, CPCL, MRPL and NRL also market petroleum products in very small proportions. The most of the market share in domestic market belongs to OMCs, and RIL and EOL have only little market share. It is to be noted that market share of private companies has been increasing since 2012-13 and quite rapidly. The major boost for this increase came from petrol price deregulation in June 2010 and diesel price deregulation in October 2014.

Demand-supply Imbalance

India has overall surplus refining capacity i.e. total production of all the refined products exceeds the total consumption. Product-wise, there is deficit of LPG and surplus of other major products viz. Petrol or MS (Motor Spirit), Diesel or HSD (High Speed Diesel), SKO (Superior Kerosene Oil), ATF (Aviation Turbine Fuel) and Naphtha. Indian refiners have been investing in up-gradation projects with the aim to convert low value heavier oil fractions to high value light and middle distillates. This has resulted into surplus of MS, Diesel, SKO, ATF and Naphtha. However, deficit of LPG is primarily due to rapid growth in its consumption.

Date of Publication: 17 July, 2017

About Author: Satyendra Kumar Singh, B.Tech. (Chemical Technology) + M.B.A., is proprietor of Satsha Management Services-an award winning design engineering and management consulting company (www.satshamanagement.com). He possesses approximately 30 years’ experience in engineering consultancy in process and energy industries. Satyendra has authored several papers on energy, business and management, which have been published in some renowned journals/magazines such as ‘Chemical Engineering’, ‘Process Worldwide’, ‘Modern Manufacturing India’. He may be reached at satyendra.singh@satshamanagement.com, Ph. +919811293605.