Strojniski vestnik - Journal of Mechanical Engineering 56(2010)x, StartPage-EndPage UDC 627.325.6:621.65 Paper received: 00.00.200x Paper accepted: 00.00.200x Calculations of the Unloading Operation in Liquid Cargo Service with High Density on Modern Product and Chemical Tankers Equipped with Hydraulic Submerged Cargo Pumps Andrzej Banaszek1, 2 - Radovan Petrovic 3 1 Technical University of Szczecin, Faculty of Maritime Technology, Poland 2 Maritime Academy of Szczecin, Faculty of Economics and Transport Engineering, Poland 3 University of Kragujevac, Faculty of Mechanical Engineering Kraljevo, Serbia The influence of different liquid cargo density on flow and drive characteristics of hydraulic submerged cargo pumps applied on modern product and chemical tankers is presented in the paper. Main parts of hydraulic structure of a/m system and cargo pump torque controller functions are described. The equation is given for the dependence of discharge rate at service of liquid cargoes (of the same viscosity but different density), based on existing drive and flow characteristics of cargo pumps, prepared for basic cargo. ©2010 Journal of Mechanical Engineering. All rights reserved. Keywords: hydraulics, submerged cargo pump, flow and drive characteristics, liquid cargo density, product and chemical tankers 0 INTRODUCTION The cargo loading area of modern product and chemical tankers as ships used for the transport of liquid petroleum-based and chemical cargoes is divided into many separate, smaller cargo tanks. This structure is the result of strong competition on the freight market of liquid cargoes sea transport. However, to an average harbour many different liquid cargoes are usually transported in small quantities. Therefore, on modern product and chemical tankers each cargo tank is equipped with a separate cargo pump. This cargo system concept enables simultaneous transport of many different liquid cargoes in one sea voyage. It also allows the increase of the ship competitiveness on the freight market and its profitability in normal exploitation. A product tanker B573_I/2 type m/t 'Hambisa' built in Szczecin Shipyard SA serves as a good example of such ships. The ship, classified by the International Classification Society - Lloyd's Register (UK), as "Oil and chemical tanker - ship type 2", was designed for the transport of different petroleum based cargoes such as: crude oil, fuel oils, xylene, toluene, lubricating oils, etc. This tanker has the cargo area divided into 20 separate cargo tanks with varied capacity from 1447.1 to 2988.2 m3 and two slop tanks with the capacity 687.2 to 712.1 m3. Every cargo tank is equipped with separate submerged cargo pump, centrifugal 1-stage type. As they were installed in a dangerous area, their power supply was designed in a hydraulic way, because it is safer than the alternative solution with electric motors. The distances between cargo pumps and other hydraulic receivers mounted on the deck are not too long. Therefore, individual hydraulic feeding systems are in this case, too expensive [1] and [2], and are not applied in the shipbuilding practice [3] and [4]. As a more economical option, the hydraulic central loading system is installed. However, simultaneous drive of several cargo pumps requires the total power up to 3000 kW and more [5] to [7]. Therefore, the hydraulic central loading systems mounted on the boards of modern product and chemical tankers are ranged among the greatest hydraulic systems not only in the ocean technology alone, but in the whole field of hydraulics, as well. The hydraulic system with such high total power enables a great number of hydraulic receivers to be supplied at the same time and independently. A detailed description of how the system works together with technical characteristics and structure, is presented in papers [1], [5] and [6]. There are many factors that influence the discharge flow of the cargo system. The control adjustments of the hydraulic drive are as important as the physical properties of the transported liquid cargoes. In catalogues producers of cargo pumps give only the flow performance characteristics pertaining to the case *Corr. Author's Address: Technical University of Szczecin, Faculty of Maritime Technology Szczecin, Poland, Al. Piastöw 41, andrzej.banaszek@ps.pl of pumping a standard, basic, liquid cargo, which, typically, is the fresh or sea water. This creates a problem for the designers of cargo installations and fuel terminal and for the tanker's operation crew. In the literature there are not any mathematical algorithms for calculation drive and flow characteristics in the case of real liquid cargoes. The results of research given by the Hydraulic Institute New York, prepared in nomographs form [8] are the most significant. Other authors like Karrasik et al. [9], Lobanoff and Ross [10], Troskolanski and Lazarkiewicz [7] and others ([12] to [14]) take advantage of these nomographs. Such an approach is impractical because it demands a manual execution of drive and flow parameters calculations by deck officers and fuel terminal staff supervising the unloading operations of tankers. This situation can lead to errors in the staff service, which can be extremely dangerous in exploitation practice. In this paper we present the influence of liquid cargo density on the change of flow of cargo pumps and drive characteristics. The resulting change of the flow of the cargo system, in the case of shipment of some different liquid cargoes, is also shown in the paper. The presented computing equation can also be helpful for more effective planning of works in harbour fuel terminals. 1 DENSITY RANGE OF THE STANDARD LIQUID CARGOES IN SEA TRANSPORTATION MARKET One of the most important technical properties of liquid cargo is its density. Table 1 shows the density of the most common liquid cargoes [15] to [18]. Standard liquid cargoes in sea transport are characterized by a large variability of density. Generally, petroleum based cargoes usually have density below fresh water density level, commonly treated as the basic cargo in maritime area comparisons (from about 667 kg/m3 for naphtha). However, the majority of chemical liquid cargoes, especially acids, are characterized by much higher density reaching about 2000 kg/m3. An example: the density of sulphuric acid (98% liquid at temperature 20°C) reaches 1830 kg/m3. Also Strong Sand Dirty Crude Oil has much higher density than water. Table 1. Density of standard liquid cargoes on sea transport freight market_ Name of liquid cargo Density [kg/m3] Temperature [0C] Crude Oil, Arabian Heavy, Ras Tannura, Saudi Arabia 887 37.8 Crude Oil, Wilmington, Long Beach, California 933 37.8 USA Crude Oil, Quiri, Carpito, Wenezuela 959 37.8 SOR Heavy Fuel Oil (HFO) 940 15 SOR Light Fuel Oil (LFO) 830 15 Gasoline, Vehicle 710 15.6 Vegetable Oil (Oliva) 910 25 Mobil VI = 146 859 40 Hydraulic DTE11M Oil Mobil VI = 141 879 40 DTE15M Gear Oil, Mobil VI = 140 75W90 859 40 Delvac 1MX2T Mobil VI = 139 80W140 870 40 Engine Oil, Vehicle Mobil 1 10W-30 VI = 147 860 40 Glycerine 1270 20 Residual Fuel Oil 970 40 Sorbo110 Methanol 790 20 Toluene 870 20 Benzene 900 20 Hydrochloric Acid (Liquid) 30% 1161 20 Sulphuric Acid (Liquid) 98% 1830 20 Such wide density range must be taken into account by an engineer designing ship cargo installation and by deck officers who supervise the unloading operation and the suitability of parameter settings of hydraulic drive systems. It may have an important influence on the shipment operation strategy and service time at fuel terminal. In case of large changes in temperature (i.e. when liquid cargo is heated), for precise calculations of flow, it is necessary to take into account the changes of the liquid cargo density as the function of temperature. For some liquids they are shown in Fig. 1. The mentioned occurrence can be described with the following simplified Eq. [16]: Pi = Po • 1 + ßt • to 1 + ß, • ti (1) where pi is liquid cargo density at new temperature t1 [kg/m3], p0 is liquid cargo density at basic temperature t0 [kg/m3], is koefficient of liquid cargo cubical expansion [-], ti is new temperature of liquid cargo [°C], t0 is basic temperature of liquid cargo [°C]. Density changes in relation to the temperature are linear. However, the speed of changes is different in relation to the varied kinds of cargoes. The above mentioned density changes for some kinds of liquid cargos as fresh water, benzene, glycerin, sulphuric acid and ethanol (alcohol) are presented in Fig. i. Fig. 1. Density changes of some popular liquid cargoes depending on temperature 2 DESCRIPTION OF HYDRAULIC SUBMERGED CARGO PUMPS USED ON MODERN PRODUCT AND CHEMICAL TANKERS Cargo pumps used on modern product and chemical tankers are usually of one stage centrifugal type. They are prepared for direct installation inside the cargo tanks. The structure of these pumps with hydraulic drive is shown in Fig. 2. [18]. The main elements of the pumps are: the head of pump, the concentric hydraulic lines with cargo discharge pipe and the deck trunk with a hydraulic control block and connection ports to the cargo deck installation, and other (e.g. hydraulic) service installations. In the head, situated in the lower part of the pump, there is an impeller driven by the hydraulic motor. As a Fig. 2. Typical cargo pump in cargo tank construction rule, in this type of cargo pumps, the high speed axial piston hydraulic motors of constant displacement are mounted. Usually, due to the required high service reliability and extremely difficult working conditions inside the cargo tanks, there are used motors of A2FM type made by Bosch Rexroth, Germany [20]. The power of standard cargo pumps reaches the level of approx. 200 kW. The hydraulic motor is supplied by means of a concentric pipe system, in which hydraulic oil flows from the control block mounted on the deck trunk. Such a construction, with the hydraulic drive motor localized in the lower part of the cargo pump, makes it possible to avoid an excessive noise and vibrations of the impeller drive shaft by reducing its length to a minimum. Usually, these vibrations in classical long-shaft cargo pumps with electric drive are caused by a bad state of the shaft bearings and an unbalanced long-drive shaft. The liquid cargo, pumped by the impeller, flows through the separate cargo pipe mounted in the pump structure to the deck trunk connection port. At the end of the cargo installation, near the pump, a cargo stop valve is installed. It is used to cut the pump off from the rest of the cargo system in the case of pump damage. A hydraulic motor, usually axial-piston type with Calculations of the Unloading Operation in Liquid Cargo Service with High Density on Modern Product and Chemical 3 Tankers Equipped with Hydraulic Submerged Cargo Pumps fixed displacement qs, is directly mounted in the lower part of the cargo pump, in the so-called pump head. In this way the length of impeller drive shaft between the drive motor and the rotor is minimal. The detailed description of the construction of the typical submerged cargo pumps with hydraulic drive, are presented in [1], [5] and [6]. As a lot of cargo pumps and other hydraulic energy receivers are installed in a small area limited by the size of the open deck, the popular way of their supplying is hydraulic central loading system. The construction of common hydraulic power pack of high power, hydraulic main lines running along the deck and consisting of pressure line (P), return (R) and leakage line (L; it does not always appear), enables a more economic and flexible powering. To these main lines all hydraulic receivers are connected in parallel way. On the inlet to the cargo pump connection ports hydraulic controllers constant-torque type are installed and they enable a stabilization of cargo pump discharge rate at variable load. The concept of the constant-torque controlling of the cargo pumps is described in [4], [21] and [22]. The detailed description of the main elements as: central loading installation, main power pack unit, auxiliary pump unit, hydraulic control distributor assembly, and oil heating unit or hydraulic oil storage tanks, is given in [1], [5] and [23]. The way cargo pumps are supplied and controlled has a significant influence on their result flow characteristics. As mentioned above, particular cargo pumps are parallelly connected to the main lines of the hydraulic central loading system. Important parameters of a given cargo pump discharge control are: main power pack unit working pressure pG adjusted by means of p = const controller (installed in power pack room) and pressure drop adjustment Aps in a constant torque controller of a cargo pump. Only by means of these two parameters can a deck officer managing ship unloading process adjust the discharge rate. The flow and drive characteristics of a typical submerged cargo pump, powered from hydraulic central loading system, is shown in Fig. 3. The shown characteristics are prepared only in relation to fixed properties of the basic cargo. This is usually fresh water, characterized by density equal 1000 kg/m3 and kinematical viscosity 1 • 10-6 m2s-1. 300 3 250 a 2oo . 100 50 Aps 24C bar Ap. = 200 bo r Ap, = 17 5bar Aps = 1 0 bar 2 5 50 75 1C 0 1 5 IE 0 17 5 200 2£ 5 2E 0 27 5 3C Cargo pump flow [m3/h] a) b) Fig. 3. Typical submerged cargo pump flow and drive characteristics a) Motor pressure drop depending on cargo pump flow b) Cargo pump load pressure depending on cargo pump flow 3 ANALYSIS OF THE PROBLEM For the analysis of centrifugal pump activity the Euler equation can be used. In accordance with this formulation [24] it can be assumed that torque value M0 on the drive shaft of cargo pump rotor is proportional to liquid cargo density: Mo=KpPc (2) where Kp is proportional coefficient [m5/s2], pc is liquid cargo density [kg/m3]. Drive torque value of axial-piston hydraulic motor driving cargo pump impeller, can be determined by means of Schloesser's equation in the following way: ms = -¿Ps-cmf - n ■APs-v (3) P-ns 4n qs -AMc where MS is performance torque on motor drive shaft [Nm], qS is hydraulic motor displacement [m3], nS is cargo pump rotation velocity [rpm], p is hydraulic oil density [kg/m ], Cmf, CmM, Cmh are proportional coefficients experimentally determined [-], AMC is constant loss torque in hydraulic motor [Nm]. According to Eq. [3] the can be simplified to the following form: MS = ■ ApS - AMS = Mst - AMS 2n (4) where MST is theoretical torque on motor drive shaft [Nm], AMS is total loss torque in hydraulic motor [Nm], and next to: qS MS * Mst =^ ■APs . 2n (5) Thus, comparing these dependencies, Eqs. (2) and (5), it can be assumed that hydraulic pressure drop in the hydraulic motor of a cargo pump is proportional to liquid cargo density: M0 = Ms , = P (6) APsc Pc where ApSW is hydraulic oil drop pressure in pump motor in case of water discharge [bar], ApSC is hydraulic oil drop pressure in pump motor in case of cargo discharge [bar], ApS is hydraulic motor pressure drop [bar], pw is water density [kg/m3], pC is liquid cargo density [kg/m3]. Hydraulic oil pressure drops are presented in Fig. 4, in the process of hydraulic motor supply from central loading system. According to Fig. 4 the hydraulic motor pressure drop Aps is: aPs = Pi - P2 (7) where is pi inlet pressure to the hydraulic motor [ ]; it is: Pi = Pg - aPrp - aPzo - aPreg (8) pG is working pressure of the main hydraulic power pack unit [ ], aprp is pressure drop in hydraulic main pressure line [ ]: APrp = z ¿r-T p-^r + d. Zz-p j=1 n -d,2 (9) where ApZ0 is pressure drop in ball valve (we can neglect a/m value as small ref. to others ApZ0 & 0), Apreg is pressure drop in hydraulic constant-torque controller, dj, Qj Uj are diameter in pipe, oil flow, oil velocity. Pressure in outlet port of hydraulic motor we can calculate as: P2 = Pzp +AP zz + aprr (10) where ^pm pressure drop in hydraulic main return line can be expressed by equation: 4 ¿Prr =ZV ^t-pu+ i1=1 dii 2 4Ö,i, ji Zj p U ji = n-d2 (11) where Apzz is pressure drop in non return valve (we can assume that for standard spring valve Apzz * 0.5 bar), pzp is adjustment pressure in support relief valve in filling up system of the main hydraulic power pack unit; the adjustment of the aforementioned support valve in typical hydraulic central loading systems amounted: pzp= 4 bar. Constant-torque controller establishing hydraulic oil flow to hydraulic motor works when this relation exists: Pi - P2